ROTOBRUSH NOT WORKING! Get error messages when trying to press spacebar for playback... help please

Potential Rotobrush Problems:

• Incompatible footage format.
• Inadequate system resources
• GPU driver problems
• AE build with a Rotoscopt build caused by an OS update
• AE build with a Rotobrush bug
• a half dozen other potential problems that occasionally pop up with certain comps

Possible Solutions

• Transcode your footage to a compatible format
• Add resources to your machine like RAM or Storage
• Update your GPU drivers / or downgrade 
• Install a different version of AE
• There are a bunch of other potential solutions

That's the best I can do without AE version, system info, OS version, and a workflow description that tells us about your source footage, its format, its frame rate, and a list of recent changes to your system to include upgrades and/or 3rd party software or plugins you may have installed.