If you're unable to generate invoices for a particular invoice frequency (Weekly, Monthly, Bi-Weekly etc) you should ensure the invoice calendars have been created for the new tax year for that particular frequency.
Each frequency is required to be created separately, and must be done before you are able to generate the invoices.
To create an Invoice Calendar:
Click Invoicing then select Create Calendar.
In the Frequency field, type the frequency required.
Fill in the details of the calendar, ensuring that the Length of Period matches your chosen Frequency.
Click Create then proceed with Invoice Generation.