This is not a built in FileMaker error message. You'll need to examine your file and see where this message was specified. From the screen shot, I'd guess that it's a custom validation error message. With custom validation messages, the message can be misleading as any validation error on the field where this message is specified will cause this message to display. So some other validation error than the "existing value" option that your error message suggests could be the source of the problem.
And if this is an "existing value" error that's causing this message to display and you know that the specific value in the field does exist in other records in the table, it's possible that the index for the field has been damaged. You might test a recovered copy and see if you get the same issue. If it does not happen in the recovered copy, you can try using advanced recover options: Copy File Blocks As Is and Rebuild Field Indexes to create a copy of the file where the only change made was to rebuild all indexes in the file.