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 "Witness version reserved for soft-fork upgrades";
        case SCRIPT_ERR_PUBKEYTYPE:
            return "Public key is neither compressed or uncompressed";
        case SCRIPT_ERR_CLEANSTACK:
            return "Extra items left on stack after execution";
        case SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH:
            return "Witness program has incorrect length";
        case SCRIPT_ERR_WITNESS_PROGRAM_WITNESS_EMPTY:
            return "Witness program was passed an empty witness";
        case SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH:
            return "Witness program hash mismatch";
        case SCRIPT_ERR_WITNESS_MALLEATED:
            return "Witness requires empty scriptSig";
        case SCRIPT_ERR_WITNESS_MALLEATED_P2SH:
            return "Witness requires only-redeemscript scriptSig";
        case SCRIPT_ERR_WITNESS_UNEXPECTED:
            return "Witness provided for non-witness script";
        case SCRIPT_ERR_WITNESS_PUBKEYTYPE:
            return "Using non-compressed keys in segwit";
        case SCRIPT_ERR_OP_CODESEPARATOR:
            return "Using OP_CODESEPARATOR in non-witness script";
        case SCRIPT_ERR_SIG_FINDANDDELETE:
            return "Signature is found in scriptCode";
        case SCRIPT_ERR_UNKNOWN_ERROR:
        case SCRIPT_ERR_ERROR_COUNT:
        default: break;
    }
    return "unknown error";
}