Billable flag does not appear on expense report line

Hi There

I have a project entered against an expense report line . There was an option to mark the line as 'billable'

For the same project and same task, on another expense report, the option is greyed out.

Previously, the project did not have a billing type associated with it. :

Is changing the project billing type from a blank to 'charge based' causing the billable field to no longer be editable?

I am trying to generate an on charge invoice for expense reports entered against the project.

I don't see any option to do that as I have created an expense based charge rule: