Why can't Drag files using Mojave

1. Go to Apple menu->System Preferences->Accessibility.
2. Select “Mouse & Trackpad”.
3. Click “Trackpad Options”.
4. Make sure that Enable dragging is turned on.
5. Click the arrow next to Enable dragging.
6. Select “three finger drag” and then click “OK”.

Does this occur for a different user, e.g., Guest? How to test an issue in another user account on your Mac.

Does this occur after starting up in Safe Mode?

Consider the possibility of a third party software conflict. EtreCheck can be used to investigate what is installed.

No problem here. Can you explain in more detail?

I am glad that I was of any help.. you're Welcome