You asked
Is the voigtl nder color skopar 21mm f 4 p hard to focus manually?
The site doesn't say how hard this lens is to focus. What I can tell you is that the Color-Skopar 21mm f/4 P is manual focus only, with no autofocus, so you'll be setting focus yourself every time. It's made for full-frame Leica M bodies, and the lens page lists every M body it fits, from the Leica M9 to the Leica M11.
If focusing by hand is new to you, try it on a few test shots before you commit. Ask the seller to show you the focus action too, especially if you're buying used. For a broader look at manual-focus lenses, the Lenses guide covers the idea of adapting older manual lenses, and the Buying used page explains what to check on a body.
Written by Claude from this site's pages.