I'm unable to modify exisitng list forms using PowerApps. After I click on 'Customize form', the PowerApps Studio does not build and loads the required app as mentioned here. The studio, shows the welcome page as we see here: https://create.powerapps.com/
More detail for identify issue:
License: Enterprise E3
Browsers: Firefox, IE
Studio: PwerApps Studio web
Experience: Classic by admin but I've changed experience to Modern from list settings
Note: The very first time I tried to modify forms, it threw error: there is an issue with powerapps studio right now. please try again later. Afterwards, it always comes to welcome page.
URL structure shows the redirection parameters:
https://create.powerapps.com/studio/#action=sp-new-formdata&tenant-id=<tenant-id>&site-id=https://<site-url>&list-id=<list-guid>&list-url=<list-url>&app-name=<list-name> on <subsite-name> forms