This error means that your mod operation library is not tuned.

Library ieee

Use IEEE.std _ logic _1164.all;

Use IEEE.std _ logic _ unsigned.all;

Use IEEE. numeric _ std.all;

Use IEEE.std _ logic _ signed.all;

Use IEEE.std _ logic _ arith.all;

There should be a missing library for operations. VHDL has been useless for a long time. What library did you forget?

If it is not for this reason, forget it.