Some of the possible reasons for receiving of "can not calculate" message are as follows:
1. The target chemical has more than 254 atoms (internal limit);
2. The target chemical contains an atom, which is not calculated by MOPAC (e.g. [Sn]);
For some chemicals MOPAC may be not finishing with successful result. MOPAC is an external application that performs quantum chemical calculations. We have little to no control over this application.