I think you custom transaction number is selected as "NO". You can check from Setting - Sales - and then customer transaction numbers. Check Image for details
Also do check the in Custom form styles which style of Invoice option you are using and click on the same and then click on content - top content there you will have to tick on custom transaction numbers.
Once both the setting are done and you try to create a new invoice you will be given an option to edit the invoice number. Now you can set the series. As in the image share by me my Invoice series is AP/2021/004 once this invoice is finished by then it will take AP/2021/005 (this is automated, we dont have to do anything)
Please let me know if any questions
Alok Shah
9326074696
