If we have an invoice with a single number sequence like INV00001, we know the next sequence will be INV00002, then INV00003 and so on.

If we want to add the sequence in the system with month and year we need to restructure the invoice with "/", to do this set the same invoice to draft status and then add the year/month/sequencenumber as show below:

The system will recognize the "/" term as year and month.

In addition be careful to add the month as the system detects the number of the month accordingly for example if we put month 1, Odoo will detect a difference and will notify us when we try to confirm