Got a reply from Odoo's support team: "The problem you're encountering is due to a bug in Odoo that's currently affecting the GMC's usability. Our bugfix team is aware of it and is working on a fix. I'll let you know as soon as I have news about its deployment." So just need to wait it seems.
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Did you see the draft documentation at https://github.com/odoo/documentation/pull/13957 - it might be easier to review it by logging into https://runbot.odoo.com/runbot/r-d-1?search=saas-18.3-ecommerce-google-merchant-center-lman (click the arrow pointing to the door icon) and then search for GMC
If you can't work it out, contact Odoo Support at odoo.com/help