While some protocols require value attached to OP_RETURN outputs, this otherwise seems like a mistake.

Setting the value to 0 should really be done by the wallet/script creating the transaction (like createrawtransaction does for `data` outputs). However, as an additional check, similar to the high-fee check, sendrawtransaction could also check for non-zero value OP_RETURN outputs.

This should be configurable by a new RPC argument.

See also https://transactionfee.info/charts/output-opreturn-amount/

#### Useful skills:

* Bitcoin Core RPC code
* Transaction output types
* Bitcoin Core functional tests

#### Want to work on this issue?

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return Untranslated("Specified sighash value does not match value stored in PSBT");
        case TransactionError::MAX_FEE_EXCEEDED:
            return Untranslated("Fee exceeds maximum configured by user (e.g. -maxtxfee, maxfeerate)");
        case TransactionError::EXTERNAL_SIGNER_NOT_FOUND:
            return Untranslated("External signer not found");
        case TransactionError::EXTERNAL_SIGNER_FAILED:
            return Untranslated("External signer failed to sign");
        case TransactionError::INVALID_PACKAGE:
            return Untranslated("Transaction rejected due to invalid package");
        
    }
    assert(false);
}
bilingual_str ResolveErrMsg(const std::string& optname, const std::string& strBind)
{
    return strprintf(_("Cannot resolve -%s address: '%s'"), optname, strBind);
}
bilingual_str InvalidPortErrMsg(const std::string& optname, const std::string& invalid_value)
{
    return strprintf(_("Invalid port specified in %s: '%s'"), optname, invalid_value);
}
bilingual_str AmountHighWarn(const std::string& optname)
{
    return strprintf(_("%s is set very high!"), optname);
}
bilingual_str AmountErrMsg(const std::string& optname, const std::string& strValue)
{
    return strprintf(_("Invalid amount for -%s=<amount>: '%s'"), optname, strValue);
}