Try the following method:

1.- WIN+R to open Run.
2.- Type: Shell:AppsFolder and press Enter
3.- It will show the "Applications" folder with all the programs installed in the system. Including UWP.
4.- From that folder drag and drop the selected program into the dock.

NOTE
As you can see in the Windows folder itself some applications are shown without icon.But access from the dock works.
Simply in those that do not show icon, right click and in Properties of the element > Change Icon
In Properties > File = the AppUserModelID (AUMID) of the UWP application is displayed.
AppUserModelID:Clipchamp.Clipchamp_yxz26nhyzhsrt!App

Solving a problem where none exists / where issue has long been addressed....

I did ask how to open the AllApps folder.

Anyway, tried dragging Paint here (23H2 now) from that folder to the dock, no issues. Sounds like the problem is in the original OP's system.

Sorry¡¡¡¡
I assume that as Moderator you can remove it if you feel like it.

Yes, I could indeed but there's no compelling reason to do so, and more so in light of user 'Winstep''s reply above.

I found out another thing. Rightclicking the running app icon and choose "Pin to dock" from the popup menu fails for Windows Store apps.



:Moderator's note: Spam removed. :/Moderator's note: - nexter

Ok, give me an example of a Windows Store App that Pin to Dock fails with, please.

Also, what version and build of Windows are you running?