I have a test file set up to test 2024 payroll and the only employee that cannot be checked with the button at the top of the Post column is marked as terminated but not inactive.  But I got a message about terminated employees and do I want to include them in the pay run.  If I answer No, they are unchecked, if I answer Yes, they are checked.

There is also the option Auto-Select: All Employees vs. Not yet paid on this Cheque Date.  An employee that was already paid will not be checked.

Pay Periods: All Periods, vs. selecting just one specific - if you have employees on 12/year vs. 26/year and you do not select All Periods, there may be some employees not selected.

Those are the only three options that might affect what you see that I can think of right now.

Sorry Non of the non selectable employee were set to terminated. all are active. All periods are selected.

I'm having the same problem.   No one has a clue as to why?    

See the notes from this link:  Payroll Cheque Run not selecting all employees  

This is a known bug and Erzsi has provided a work-around.

when I advanced my using date to the end of the month it resolved.