This graphics configuration does not support hardware acceleration, which is required to run iMovie.

Hi! I solved this issue doing a SMC&PRAM

Which version of iMovie are you trying to run?

Make sure, that you did not accidentally into Safe Mode (see: Try safe mode if your Mac doesn't finish starting up - Apple Support). Then the advanced features of your Graphics card will be disabled and some applications will complain.

Which version of iMovie are you using? Did the problem start directly after the El Capitan upgrade?

The error message indicates a problem with the graphics card, but your iMovie version should be compatible even with older cards. iMovie 10 is less restrictive with respect to graphic cards than the previous iMovie '11. (https://www.macnn.com/articles/13/11/20/also.improves.reliability.when.updating. projects.events.from.previous.versions/)

Try to boot into safe mode, then back to normal mode to clear some caches, (see: Try safe mode if your Mac doesn't finish starting up - Apple Support)

And then try again to launch iMovie.

I hope one of our hardware specialist will chime in.

The only limitation I can find in the iMovie technical specification relates tto 4k videos.

Does the project you are trying to open contain 4k video?

I'm not trying to open any specific project, I'm just trying to open the program itself.

But it will try to open your previous iMovie Library. So the question is, if the video in that library is compatible.

You could try to launch iMovie while holding down the options key. That will let you create a new iMovie library.