Hi,
Thank you for reaching out to the Google Ads API support team.
From the provided sample code, I could see that you are getting the error "UNSUPPORTED_FIELD_TYPE". This means that the field type 'MARKETING_IMAGE' is not supported to be added directly through asset links. Please refer to this guide to know the asset types that can be linked to customers, campaigns, or ad groups through linkage objects. According to our API documentation, you can link the 'MARKETING_IMAGE' image asset directly to the Ad and you may use AdService to achieve this via the API.
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Google Ads API Team |