Welcome to the Manjaro community @CaiozZ
I was just able to drag & drop a file from Dolphin onto my Plasma desktop, which is set to Desktop layout, so maybe try switching from Folder View to Desktop.
The issue could also be caused by the type of Plasma session you are running. If you are on X11, try changing to a Wayland session. You can check your session type by running the following command in a terminal:
echo $XDG_SESSION_TYPE
You can change your Plasma session from X11 to Wayland by selecting the session type at the login screen (Wayland is the default for Plasma).
If you have set up autologin to skip the password screen, you can change to a Wayland session by opening System Settings → Colors & Themes → Login Screen (SDDM) → Behavior.
If neither of the above suggestions works, then further troubleshooting may be required.
Further reading:
As a new or infrequent forum user, please take some time to familiarise yourself with Forum requirements, and the many ways to use the forum to your benefit:
Update Announcements
The Update Announcements contain important information and a Known Issues and Solutions section that should generally be checked before posting a request for support.
System Information
Output of this command (formatted according to forum requirements) may be useful for those wishing to help:
inxi --filter --verbosity=8
or the short form:
inxi -zv8
Be prepared to provide more information and outputs from other commands whenever asked. It’s equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.
Required reading
Resources
Regards.