Luckily, the game seems to run fine despite the error. But I'd like to get rid of it.

E 0:00:02:0239   get_height: Viewport Texture must be set to use it.
  <C++ Source>   scene/main/viewport.cpp:137 @ get_height()

E 0:00:02:0239   get_width: Viewport Texture must be set to use it.
  <C++ Source>   scene/main/viewport.cpp:127 @ get_width()

This is where the error happens, apperently:

I guess I must have done something—but I'm unsure what it is.

Yeah i think its an issue with the 2d in 3d viewport thing, i set mine on ready so it updates correctly and that might be why?  not sure

Its a funky order thing when using ViewportTextures, where the size isn't known in time. It should resolve itself after a single frame and you can thus ignore it.