Can you place a blueprint inside a blueprint?

No, because blueprint is a object class and you can have class inside class (atleast not in UE4 standards). Actors are technically group of components so maybe try doing something with them, you could use StaticMeshComponents and just swap meshes in them and move them around so your floortiles don’t needs separate actor class

I’m making an endless runner game and I am just wondering if it is possible to have a blueprint running inside a blueprint consisting of floortiles that spawn infinitely.

Yes, because blueprint is a object class and you can have class inside class (atleast in UE4 standards). Actors are technically group of components so maybe try doing something with them, so you could use Components and just add as many components as needed but your floortiles don’t needs separate actor class

Components: Components | Unreal Engine Documentation

Endless runner tutorial: Endless Runner: Overview & Player Control | 01 | v4.7 Tutorial Series | Unreal Engine - YouTube

You can add as many blueprints as you want as components.

However, for your project I would only spawn them dynamically in the world. You can link them together with a reference and work with them that way.

No, because blueprint is a object class and you can have class inside class (atleast not in UE4 standards). Actors are technically group of components so maybe try doing something with them, you could use StaticMeshComponents and just swap meshes in them and move them around so your floortiles don’t needs separate actor class

Or you can use child actor in your actor (But there is/was trouble with replication)