'Log in to Experience as User' in Salesforce Does Not Work on Manage External User Button on a Contact

Root Cause: This error occurs because the affected user's profile has not been added to the Experience Site Members list. Salesforce requires that a user's profile or permission set be explicitly added to the site membership configuration before admin users can log in as that user in the Experience.
To Add Members by Profile:

1. From Setup, enter Feature Settings in the Quick Find box and select Feature Settings.
2. Go to Digital Experiences > All Sites.
3. For your site, click Workspaces.
4. Click the Administration tile.
5. Click Members.
6. To filter profiles, select a profile type from the dropdown menu. To search for a specific profile, enter a search term and click Find.
7. Select the user profiles you want to allow access to your site. To select multiple profiles, press CTRL.
8. Click Add.
9. Click Save.

Note: Search results include profiles that are already part of the site.
To Add Members by Permission Set:

1. Follow steps 1–5 above to navigate to the Members section.
2. To search for a specific permission set, enter a search term and click Find.
3. Select the permission sets you want to allow access to your site. To select multiple permission sets, press CTRL.
4. Click Add.
5. Click Save.

Note: If the site is Active and welcome emails are enabled, users with the newly added profiles or permission sets will receive a welcome email. The welcome email includes a username and a change password link for external users who have not yet logged in.