When you have unneeded pay calendars, this message appears in the Summary tab: Pay runs completed.
This can be because:
Your pay frequency has changed, from weekly to monthly, for example
A pay calendar was set up in error
If the calendar isn't assigned to any employees and you haven’t completed a pay run for it, you can remove it.
From Settings, select Pay Day Settings.
On the relevant calendar, select Manage Frequency and Remove Frequency.
When prompted, select Yes.

The workaround is to move it to the last period of the current tax year. This prevents the Pay runs not completed warning message and closes the reporting period.
You can't skip over the last period of the tax year. You'll need to process the final pay run and complete your year end before you can remove the frequency.