I get this sometimes in the designer. I usually use the shortcuts CMD+C and CMD+V to copy and paste, respectively.
You need to select the element first, then on the blue element tab, right click to copy instead of using the shortcut. My Mac makes a “doink” sound if it hasn’t copied using the shortcut.
It often annoys me, but a quick refresh of the designer sorts it out for a while.
Not sure if that’s your issue or not, hope it helps!
You also can’t copy elements from a dynamic list onto a different dynamic page without first disconnecting the fields first. It’s a good idea to have a normal page with all the styles set up so you can copy and paste from there, then connect the dynamic file.
If that’s the case though, you normally see an error in the top right hand corner telling you this.