When I create the Rollup field, it results in two fields being created - "Rollup Sample (Last Updated On)" and "Rollup Sample (State)" which are DateTime and Whole Number respectively. Selecting the Whole Number field does not give you the option to open the Rollup calculation window (See Image).
Additionally, I tried to open the rollup calculation when creating the field but the new window does not open.
I do not have this issue when creating a Calculated Field.
I can work around this issue by switching to the classic solution editor, but it might be worth logging this as a bug if it is one
You need to understand how the Rollup and Calculated fields work, they are different and have specific functionality, and create backend system fields that you don't need to modify or use, please review the following documentation
hope this will help!