Reindex - Upgrade assistant index data prior to 7.0

[elkstack1 (ELK)]

I clicked on the reindexing option on the upgrade assistant for the indexes that are critical and needs to be reindexed prior to upgrade.
I do see the reindexing failed but there is no error detailed on the response. Can you please suggest how to analyze and figure out the root cause of reindexing failed?

[elkstack1 (ELK)]

This is the error response that I saw in the upgrade assistant during reindexing documents
Error: Reindexing failed: {"completed":true,"task":{"node":"SfhPl3CqRvOl5jhg3OW3dA","id":21278982,"type":"transport","action":"indices:data/write/reindex","status":{"total":13,"updated":1,"created":12,"deleted":0,"batches":1,"version_conflicts":0,"noops":0,"retries":{"bulk":0,"search":0},"throttled_millis":0,"requests_per_second":-1,"throttled_until_millis":0},"description":"reindex from [testing] to [reindexed-v7-testing][_doc]","start_time_in_millis":1663098485550,"running_time_in_nanos":144562427,"cancellable":true,"cancelled":false,"headers":{"X-elastic-product-origin":"kibana"}},"response":{"took":144,"timed_out":false,"total":13,"updated":1,"created":12,"deleted":0,"batches":1,"version_conflicts":0,"noops":0,"retries":{"bulk":0,"search":0},"throttled":"0s","throttled_millis":0,"requests_per_second":-1,"throttled_until":"0s","throttled_until_millis":0,"failures":}}

[elkstack1 (ELK)]

I do see that the task is not found error on Elasticsearch log.