Kibana server is not ready yet suddenly

everything was fine until suddenly it appears "Kibana server is not ready yet".
i have 1 master node (master), and 1 data node(node-1). It was working fine days until i realize.

master:9200

[root@localhost kibana]# curl localhost:9200
{
  "name" : "master",
  "cluster_name" : "elasticsearch",
  "cluster_uuid" : "P4QlwvuRRGSmlT77RroSjA",
  "version" : {
    "number" : "7.6.1",
    "build_flavor" : "default",
    "build_type" : "rpm",
    "build_hash" : "aa751e09be0a5072e8570670309b1f12348f023b",
    "build_date" : "2020-02-29T00:15:25.529771Z",
    "build_snapshot" : false,
    "lucene_version" : "8.4.0",
    "minimum_wire_compatibility_version" : "6.8.0",
    "minimum_index_compatibility_version" : "6.0.0-beta1"
  },
  "tagline" : "You Know, for Search"
}

node-1:9200

[root@dev ~]# curl localhost:9200
{
  "name" : "node-1",
  "cluster_name" : "elasticsearch",
  "cluster_uuid" : "P4QlwvuRRGSmlT77RroSjA",
  "version" : {
    "number" : "7.6.1",
    "build_flavor" : "default",
    "build_type" : "rpm",
    "build_hash" : "aa751e09be0a5072e8570670309b1f12348f023b",
    "build_date" : "2020-02-29T00:15:25.529771Z",
    "build_snapshot" : false,
    "lucene_version" : "8.4.0",
    "minimum_wire_compatibility_version" : "6.8.0",
    "minimum_index_compatibility_version" : "6.0.0-beta1"
  },
  "tagline" : "You Know, for Search"
}

both node can see each other

[root@localhost kibana]# curl master:9200/_cluster/health?pretty
{
  "cluster_name" : "elasticsearch",
  "status" : "yellow",
  "timed_out" : false,
  "number_of_nodes" : 2,
  "number_of_data_nodes" : 2,
  "active_primary_shards" : 27,
  "active_shards" : 27,
  "relocating_shards" : 0,
  "initializing_shards" : 0,
  "unassigned_shards" : 27,
  "delayed_unassigned_shards" : 0,
  "number_of_pending_tasks" : 0,
  "number_of_in_flight_fetch" : 0,
  "task_max_waiting_in_queue_millis" : 0,
  "active_shards_percent_as_number" : 50.0
}

kibana installed on master node.
systemctl status kibana

[root@localhost kibana]# systemctl status kibana -l
● kibana.service - Kibana
   Loaded: loaded (/etc/systemd/system/kibana.service; enabled; vendor preset: disabled)
   Active: active (running) since Thu 2020-06-04 09:49:15 WIB; 28min ago
 Main PID: 61223 (node)
   CGroup: /system.slice/kibana.service
           └─61223 /usr/share/kibana/bin/../node/bin/node /usr/share/kibana/bin/../src/cli -c /etc/kibana/kibana.yml

Jun 04 09:49:44 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:44Z","tags":["warning","plugins","security","config"],"pid":61223,"message":"Generating a random key for xpack.security.encryptionKey. To prevent sessions from being invalidated on restart, please set xpack.security.encryptionKey in kibana.yml"}
Jun 04 09:49:44 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:44Z","tags":["warning","plugins","security","config"],"pid":61223,"message":"Session cookies will be transmitted over insecure connections. This is not recommended."}
Jun 04 09:49:44 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:44Z","tags":["warning","plugins","actions","actions"],"pid":61223,"message":"APIs are disabled due to the Encrypted Saved Objects plugin using an ephemeral encryption key. Please set xpack.encryptedSavedObjects.encryptionKey in kibana.yml."}
Jun 04 09:49:44 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:44Z","tags":["warning","plugins","alerting","plugins","alerting"],"pid":61223,"message":"APIs are disabled due to the Encrypted Saved Objects plugin using an ephemeral encryption key. Please set xpack.encryptedSavedObjects.encryptionKey in kibana.yml."}
Jun 04 09:49:44 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:44Z","tags":["info","plugins","monitoring","monitoring"],"pid":61223,"message":"config sourced from: production cluster"}
Jun 04 09:49:44 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:44Z","tags":["warning","plugins","monitoring","monitoring"],"pid":61223,"message":"X-Pack Monitoring Cluster Alerts will not be available: undefined"}
Jun 04 09:49:45 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:45Z","tags":["info","savedobjects-service"],"pid":61223,"message":"Waiting until all Elasticsearch nodes are compatible with Kibana before starting saved objects migrations..."}
Jun 04 09:49:45 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:45Z","tags":["info","plugins","watcher"],"pid":61223,"message":"Your basic license does not support watcher. Please upgrade your license."}
Jun 04 09:49:45 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:45Z","tags":["info","plugins","monitoring","monitoring","kibana-monitoring"],"pid":61223,"message":"Starting monitoring stats collection"}
Jun 04 09:49:46 localhost.localdomain kibana[61223]: {"type":"log","@timestamp":"2020-06-04T02:49:46Z","tags":["error","savedobjects-service"],"pid":61223,"message":"This version of Kibana (v7.7.0) is incompatible with the following Elasticsearch nodes in your cluster: v7.6.1 @ masterip:9200 (masterip), v7.6.1 @ node-1:9200 (node-1)"}


Any kind of respond will be appreciated so much. thank you in advance

The error in your Kibana log tells you what's wrong:

You're using an incompatible version of Kibana, it needs to be the same version as the version of your Elasticsearch installation.

The solution is either to upgrade Elasticsearch to version 7.7.0 or downgrade Kibana to 7.6.1. Good luck!

amazingly done thank you so much

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