Architecture student, how can i use UE 4.21 with student license?

You may use the Unreal Engine freely as a student. We do not require any student identification. (the engine is free for everyone to use)

As an Architecture student, I recommend you choose the Unreal Studio beta. This includes the Unreal Engine, but also includes additional features for those in the non-games industries. Again, it is free to use, and as a student, you will not be selling and distributing your project for sale anyway.

You can review the Unreal Studio EULA here: Datasmith - Unreal Engine

You can join the Unreal Studio beta here: Datasmith - Unreal Engine

Cheers