I am actually learning Scrum, and there is no recommendation for a particular app architecture.
I have the feeling that Scrum is designed to go with Domain Driven Design, and that every Sprint should map to a Bounded Context, which are better implemented with Microservice.
So I am wondering if Scrum can be fully implemented while building a Monolith ?
It depends on your definition of monolith. If you just mean a very large centralized system, this is completely possible (though one could argue against that architectural approach in general).
On the other hand, if you mean that there is a singular system designed in its entirety up-front and then built as one large deliverable, then no, Scrum is specifically an incremental approach to development and building in large phase-gates runs directly against Scrum.