The rectangle you are trying to rotate probably has a Horizontal-Vertical constraint on at lease one line. Remove that constraint and you can rotate the rectangle.
Thinking about this more, here is a real case where the app can be improved by the Shapr team by providing a better Error Message as in: Object Cannot Be Rotated Because Constrained to Horizontal/Vertical.
Yes, that would be great - the problem is that in non-trivial cases you just can’t tell why the user can’t rotate it. Although we could cover the trivial ones manually.