Thanks for connecting with me, @robbietharp !
Thank you for providing all that context and helping me understand exactly what you are facing and at what point in the set up you experience this error.
I spent quite a bit of time digging into this with our Marketplaces team and we had a thorough look into what Shopify is able to influence in the Tik Tok sales channel.
As a disclaimer, while our Support team does our best to assist with Tik Tok related issues, the channel is fully supported by their team. Shopify, on the other hand, can step in when issues are related to API. To be even more clear, general Tik Tok support is likely to be unable to help and will send you back to Shopify.
To connect with the correct team within Tik Tok please use their support portal.
I know it’s definitely frustrating to be sent back and forth, but I hope that support portal is what you need to move forward.
Please keep me posted,