Please ensure you have entered dot(,) between dash(-) and hashtag (PO-.######). I didn’t mention dot in the series for first time, and I received an error in naming as indicate in your message.
I searched the forum for a solution and in another post it was suggested to ‘update’. I ran ‘bench update’ and now i have bigger problems - the whole system (3 months of work) is broken!
The customization of number of digits is not possible, by just adding “######”, in case of “Naming Seies” field. You need to add a server side autoname function to do that.