This is usually a direct issue with the user being used in that action. It can be in multiple places, but the easiest is to go check, in your flow run and or if its hardcoded, who the UPN/Email is.
Then go to your Admin Center, to go the Environment and go to Users. Look for that user and if you do not see them then its easy, they no longer exist in the environment.
If they do not exist, is it because you added a security group???? and they are not in it??
Validate that they exist and have licenses. If they are there, click them and run the diagnostics at the top of the screen in Users, after you select them.
If they do not exist, add them back, give them licenses.
Lastly, you can verify that how you are connecting is good, verify under powerautomate.com ...More, Connections that you see their connection and if its good or bad.
But usually its simply their account is jacked up.. and missing or disabled.
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