Login As Experience User button not visible for internal Users

The "Login As Experience User" button requires all of the following permissions and access settings to be configured for the internal user attempting to use it:
Required Conditions

1. Under Setup > Profiles > App Permission — "Manage External Users" must be checked for the internal user's profile.
2. Object Level Access to Account must be set to Edit on the internal user's profile.
3. Record Level Access to Account must be set to Edit (the internal user must have edit access to the specific Account record).
4. Community Membership — the internal user must be a member of the Experience Cloud site via a permission set or profile access.

Note on Role Hierarchy Exception
"Edit" access on the Account is not required if the internal user logging in has a role in the role hierarchy that is higher than the Partner or Customer user they are attempting to log in as. In this scenario, role hierarchy access is sufficient for the "Login As Experience User" button to be visible.