PSDT always couldn't be uploaded because it is not a valid file type

Read this page:

https://helpx.adobe.com/stock/contributor/help/design-template-requirements.html

which says:

At the moment, we’re sourcing design templates from a small group of artists and agencies — we’re not accepting submissions from individual contributors at this time. Over time, we may be expanding them to include a broader group of contributors — we’ll keep you up to date in our Contributor newsletters and notifications.