The usage message states:
dwibiascorrect [ options ] algorithm ...
This way the gradient table option is ignored which leads to an error because the data isn't identified as diffusion data.

The documentations on the website states:
dwibiascorrect algorithm input output [ options ]
This way dwibiascorrect works as expected.

• OS: Ubuntu 20.04.2 LTS
• MRtrix 3.0.2-65-g6e74de10