While you were trying to update the key Flex field in UI they were also being updated through File Import, and hence the error had occurred. This is an accepted behavior. The row was locked for updation by one application when another one was trying to update it. You should not modify the Opportunities through UI, when an import job is modifying them in the background.

You should not modify the Opportunities through UI, when an import job is modifying them in the background.

Please try after some time you when the job is completed you should not get this error.