Hello,
Please re-download HikaSerial in order to get a patch regarding Joomla4.
You will have to delete your current PDF serial config and create a new one ; it will fix the saving of the "type".
Regarding the generation, if the plugin can't access the PDF template, it will be a problem. I recommend you to let the field empty for the moment ; your PDF won't have a template but it will create you a PDF from scratch.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.