Hi,
What steps have you tried so far?
What's your account type? Exchange, POP or IMAP?
Can you change the flag status on the web mail?
How did you try to mark the messages as complete? Did you right click on it and selected Follow Up > Mark Complete?
Based on my research, normally, you will get this error if you change the message during its synchronization with the server. Normally, we can wait a few second or switch messages and then click the flag icon again to see the results.
If you keep receiving this error, we can repair Outlook data file in case account data file was corrupted. Please refer to the status in
this link.
Besides, please collect the detailed version information for Outlook via File > Office Account for further research.
Any updates, please feel free to post back.
Regards,
Perry
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