sub divide probelms

Car Game In Blender

i need help when i click subdivide it says
unable to complete action with multires enabled whats that mean i folloew a tutoriAL FINE up to that point as then i have errors

heres the tutorial

This should be a simple fix. All you should need to do is go to the editing button (f9) and find the multires tab. Click on the apply multires button. Then try the subdivide again.