Hello,
The error you have is related to the TCPDF library that is include in HikaSerial package.
While HikaSerial is using the library version 6.2.12 ; it is possible that a newer version would fix the issue if they worked on the PHP8 compatibility.
github.com/tecnickcom/TCPDF
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.