how to fillet between two circle

Are you trying to fillet between two cylinders, or two sketched circles?

OK, a LOT more information needs to be given.

Do you mean 2 cylinders, like this?

The keys:

You can't fillet between 2 separate bodies, so you need to connect them somehow.  This is why I added the little bridge between them.

Use Face fillet.

Trim the overlapping lines of the two circles & you will be able to fillet. If they are not overlapping, it will be best to draw two arcs rather than two circles as you will need to break the circles somehow to allow a fillet to work. If you don't want to use a the fillet command, you could just add an arc between the two circles, add tangent constraints & trim after.....& fully define of course