Reduce::mdom
FindInstance::mdom
A warning message is generated because specification of a modulus implies a domain, so the domain specification (the third argument in Reduce) is unnecessary:
Reduce[x ^ 3 == y, {x, y}, Integers, Modulus -> 5]The warning message can be avoided by removing the domain specification:
Reduce[x ^ 3 == y, {x, y}, Modulus -> 5]