bad cut

Hello, my cnc when cutting the cube is 19.5 in 20mm and the circle is 20.5mm.
What is possible

Make sure your end mill is actually the diameter it is supposed to be.

Outer or inner diameter? Those are somewhat close but, if it is a light cut in wood something may be off with your cam or squareness. Doesn’t sound bad enough for skipped steps.

Agreed, probably not skipped steps, verify the bit diameter, if that checks out make sure you are using a finish pass on your cuts.

How to check finish pass on your cuts.

In estlcam there is a box for finish tool, select the same tool you are using for the cut and I would try a 0.5mm allowance to start with.