For example, ‘circle’ is a concrete noun and ‘circularness’ is an abstract noun. Though can a circle be circularness and circularness be a circle?
‘Circularness is a circle.’
I have found the word “circularness” in several online dictionaries other than Wiktionary, though it definitely isn’t something I would use. Circularity (as suggested by Kate Bunting) sounds better, but I think roundness is more common.
When you say:
A circle is circularness.
this is wrong. You need an adjective to describe the circle, not a noun. You should change this to
A circle is circular.
A circle is round.