Hi @matthu
I suspect it is with licensing. Rather than seeing from Dynamics 365 licensing point of view. Check whether you have Power Apps licensing.
1. Power Apps Per-App licensing includes a portal. If you have more than one powerapps you are accessing, most likely you need more than one license
2. Power Apps Per-user licensing which will have unlimited access to all powerapps and portals.
The easiest way to check your licensing is
1. Log in to https://make.powerapps.com/
2. Click on the Settings gear icon > plans
Additional
1. Go to power platform admin centre > choose the right environment
2. users > roles >make sure you have a system administrator or system customiser license. ( or both)
If you have everything set up properly and still getting errors. Make sure you press ctrl + F5 on the web pages (Portal management) screen. This will resolve your issue.
Hope it helps.
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