How to enable 'Actions' menu items on absence certifications?

Summary:

How to enable 'Actions' menu items on absence certifications?

Content (required):

We have an employee where we need to override the 'Expiration Date', 'Revised Payment Start Date' and 'Revised Payment End Date' fields on the certifications on their sickness absence. I can see that options exist to do this exact task on the 'Actions' menu but all options are greyed out.

How do these get enabled? Is there specific setup we need to do on the absence certification? Most of the configuration options on the certification seem to be locked down but that may be because it's already in use?