ReSA Total - Register Summary Information must be available error in ReSA v19

Following error is received when clicking "Next" button on page 4 (after adding realms) in the total screen. This error is received when creating "Accountable" and "Accounted For" O/S totals at Register balancing level.

System option balancing level is set to Register. I would like to understand why this error is received and what settings to be checked?

User guide states the following -

"The Combined Total Details window allows you to combine two existing totals. In order to be combined, the totals must be compatible. To be compatible, totals must be defined at the same balancing level and have the same roll-ups." I assume components and O/S should be at the same balancing level.