When registering a custom widget, "The URL does not reference an AppBuilder Extension"

[AbbyYoung]

I've hosted a custom widget on our web server, and I'm able to access the code in manifest.json path (see below).

Now I'm trying to register a custom widget in our portal (ArcGIS Enterprise 10.9.1).

And I got this error "The URL does not reference an AppBuilder Extension". Can anyone help?

[A_Wyn_Jones]

Can you check whether your Portal can reach that URL? the checkUrl function is ideal for this and can be accessed like so:

Go to https://portal.domain.com/portal/sharing/rest > Login > Home > Portals > Self > CheckURL

You can also use this URL programmatically:

https://portal.domain.com/portal/sharing/checkUrl.jsp?url=<url>