https://github.com/bitcoin/bitcoin/blob/57a8fa9c3b4452ef7a9d9ed4b957b3888f542fb0/src/script/script_error.cpp#L60-L61  
It should be `Non-push operators are not allowed in signatures` or `Only push operators allowed in signatures`.
return "No error";
        case SCRIPT_ERR_EVAL_FALSE:
            return "Script evaluated without error but finished with a false/empty top stack element";
        case SCRIPT_ERR_VERIFY:
            return "Script failed an OP_VERIFY operation";
        case SCRIPT_ERR_EQUALVERIFY:
            return "Script failed an OP_EQUALVERIFY operation";
        case SCRIPT_ERR_CHECKMULTISIGVERIFY:
            return "Script failed an OP_CHECKMULTISIGVERIFY operation";
        case SCRIPT_ERR_CHECKSIGVERIFY:
            return "Script failed an OP_CHECKSIGVERIFY operation";
        case SCRIPT_ERR_NUMEQUALVERIFY:
            return "Script failed an OP_NUMEQUALVERIFY operation";
        case SCRIPT_ERR_SCRIPT_SIZE:
            return "Script is too big";
        case SCRIPT_ERR_PUSH_SIZE:
            return "Push value size limit exceeded";
        case SCRIPT_ERR_OP_COUNT:
            return "Operation limit exceeded";
        case SCRIPT_ERR_STACK_SIZE:
            return "Stack size limit exceeded";
        case SCRIPT_ERR_SIG_COUNT:
            return "Signature count negative or greater than pubkey count";
        case SCRIPT_ERR_PUBKEY_COUNT:
            return "Pubkey count negative or limit exceeded";
        case SCRIPT_ERR_BAD_OPCODE:
            return "Opcode missing or not understood";
        case SCRIPT_ERR_DISABLED_OPCODE:
            return "Attempted to use a disabled opcode";
        case SCRIPT_ERR_INVALID_STACK_OPERATION: