Rotobrush dosent work

Try this:

After trimming your layer so you are not working with frames that are not in the final edit, pre-compose moving allattributes to the new comp but do not open the new comp. Now select the nested comp, that's the pre-comp you just created, and right click or use the layer menu to Open Layer. Select the rotobrush tool, click once inside the area you want to mask, then immediately preess Caps Lock to stop rendering. Check the Effects Control Panel and change the version of Rotobrush from 2.0 to 1.0. Release the caps lock and conplete your roto work, then freeze, then pre-compose the nested comp again/ moving all attributes to the new comp, open the new comp and choose Pre-render from the Composition menu to create a new lossless with alpha video from the comp and replace it in your main comp. 

Right now Rotobrush version 2 is not working. That workflow should solve your problem. Bug reports have been filed.

If you are using macOS, there is a problem with BigSur 11.5.2 that breaks the rotobrush. Not sure if you are. using macOS or windows. Could you share with us more info and specs about your computer?