Is there any problem with compatibility...?

Elastic Stack Elasticsearch
1

We are using Elasticsearch 6.5.2 + Kibana 6.5.2 in Ubuntu 18.04.

Among them, even if you upgrade the Ubuntu version to 20.04
Is there any effect on the use of Ubuntu 20.04 + Elasticsearch 6.5.2 + Kibana 6.5.2 ...?

( Support Matrix | Elastic )
Is there any problem with compatibility...?

I have upgraded the Ubuntu version to 20.04 now, but there is noproblem with Elasticsearch and Kibana operation.

2

According to the support matrix Ubuntu 20.04 is not supported for the very old version you are using. This means that it has not been tested but it is possible that it may still work. There are no guarantees though. Note that the version you are using is very old and has been EOL a long time so I would strongly recommend that you upgrade to the latest version.