If your team have already done so, would you mind sharing with me your developer app ID and the HubSpot portal that you're looking to send the transactional email from?
Thanks for sharing your developer app information. However, could you also share with me the HubSpot portal ID that you're maintaining the transactional email on?
In this case, it looks like 7289724 is a test portals and we do not automatically provisioned test portal with the Transactional Email add on in order to prevent abuse. I'll send you a DM for more information.