This is a current bug, see advisory message SP207180 that is posted in the Office 365 admin center and suggested workaround:
Title: All admins may be unable to address access requests for classic
SharePoint Online sites in admin center
User Impact: Admins are unable to approve or deny access requests to
classic sites in the SharePoint Online admin center.
More info: The approve and deny buttons aren’t populating.
As a workaround, admins can sort the users requests using any field
and the accept/ decline buttons will populate.
Current status: We've validated a code fix for this issue and expect
deployment to complete by Monday, March 30, 2020.
Scope of impact: This issue may affect any admin attempting to approve
or deny user access requests to classic SPO sites through the SPO
Estimated time to resolve: We expect deployment to be completed on
Monday, March 30, 2020.
Root cause: A recent deployment caused the accept/ decline button not
to populate for admins within the SPO admin center.