How big is the radius that cannot be applied? If the fillet of the outer edge intersects with the inner edge, it may end up in the error message of adjoining edges.
Try to apply the fillet on the inner edge first, then the outer edge and let us know how it works.
It should be 20mm, but I tried very small as well when things failed.
Actually when one tries to apply too large radius one gets another error (which makes sense I suppose).