Erik Kostson
Ansys Employee
Well, you did not check properly. Chapter 14.2 Inertia relief in the help manal (Theory Reference part).

As we said inertia relief can only be used from 2021 R2 and onwards.

Can you please explain what you mean with this in the your message above, and with a different analysis method. It is not really clear so please explain what you want to do, but if it is inertia relief in a nonlinear analysis (large deflections), as we said it can only be done in/from 2021 R2.
Thank you