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 "Operation not valid with the current stack size";
        case SCRIPT_ERR_INVALID_ALTSTACK_OPERATION:
            return "Operation not valid with the current altstack size";
        case SCRIPT_ERR_OP_RETURN:
            return "OP_RETURN was encountered";
        case SCRIPT_ERR_UNBALANCED_CONDITIONAL:
            return "Invalid OP_IF construction";
        case SCRIPT_ERR_NEGATIVE_LOCKTIME:
            return "Negative locktime";
        case SCRIPT_ERR_UNSATISFIED_LOCKTIME:
            return "Locktime requirement not satisfied";
        case SCRIPT_ERR_SIG_HASHTYPE:
            return "Signature hash type missing or not understood";
        case SCRIPT_ERR_SIG_DER:
            return "Non-canonical DER signature";
        case SCRIPT_ERR_MINIMALDATA:
            return "Data push larger than necessary";
        case SCRIPT_ERR_SIG_PUSHONLY:
            return "Only non-push operators allowed in signatures";
        case SCRIPT_ERR_SIG_HIGH_S:
            return "Non-canonical signature: S value is unnecessarily high";
        case SCRIPT_ERR_SIG_NULLDUMMY:
            return "Dummy CHECKMULTISIG argument must be zero";
        case SCRIPT_ERR_MINIMALIF:
            return "OP_IF/NOTIF argument must be minimal";
        case SCRIPT_ERR_SIG_NULLFAIL:
            return "Signature must be zero for failed CHECK(MULTI)SIG operation";
        case SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS:
            return "NOPx reserved for soft-fork upgrades";
        case SCRIPT_ERR_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM: