When you impersonate a user and then try to re-impersonate your own user account, the Impersonate User popup window shows an error Failed API level ACL Validation
Missing a required role, snc_internal, for the RESTAPIProcessor Access Control (ACL) record.
1. Go to System Security > Access Control (ACL)
2. Filter for Name is RESTAPIProcessor
3. Open the matching record
4. Under the Requires Role related list, if you see a role that has a blank Sys user role, double click the blank value and in the suggestions popup, find and select the snc_internal role. Click the green checkmark to save the change.
5. Save the ACL record.
After performing these steps, you should be able to impersonate other users and re-impersonate yourself without getting the error.