I have some troubles with deploying .sppkg solution. I tried to add somehow application to my site but i can't deploy it even in developer site. I tried to add this app to AppCataloge in admin site, but add button doesn't exist (I am an admin). I can debug project using local/hosted workbench.
The problem, why I couldn't add apps was related to Site Collection. I found in my site collections list -> https://mypage.sharepoint.com/sites/App_Catalog -> and I added my account to admins of this page.
@jussi-palo - I apologize for that I could misdirect you (I thought that sharepoint admin is also an admin of App_Catalog)
But still I can't add any apps through Dev site (and Extension-type apps are always invalid).