Reindex - Upgrade assistant index data prior to 7.0

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?

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":}}

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