This warning is alerting you that the structural force convergence value used in the Newton-Raphson Procedure is very small. This can happen when applied loads are very small or zero. It is just a warning. If your model convergences successfully, and everything balances, you can probably ignore it. If you are having convergence troubles, the warning is a clue that this might be part of the problem.
The MINREF field of the CNVTOL command is a default minimum value designed to prevent the code from attempting to converge on a nearly zero tolerance. You can read more about the Newton-Raphson Procedure in Section 14.10 of the MAPDL Theory Manual. Refer also to the online docs for CNVTOL Command in the MAPDL Commands Manual.