Hi,
the auxiliary basis set for those elements is included in the basis set library, the problem lies within the meanwhile obsolete rimp2prep. Please use define instead to assign the cbasen:
In the last menu of define, enter mp2 and then cbas. You will get a menu very similar to the usual basis set menu. define is usually able to assign the correct auxiliary basis set automatically for Dunning basis sets. That is for rimp2 calculations. You can enter cc2 instead of mp2, then again cbas. In the section ricc2 enter mp2 for RI-MP2 energy calculations with the module ricc2. For geometry optimizations, you have to enter geoopt mp2 in addition.
Regards,
Uwe