Your shader code is broken. The varying registers are used to pass data from the vertex shader to the fragment shader, such as tex coords. If you don’t set a register in the vertex shader, and try to read it in the fragment shader, then that is the error you would expect.
edit: and it’s varying 3 it’s complaining about, so that would be I think v2, as they are zero-indexed. Should be easy to spot in your AGAL.