Hello,
Please understand that if we need access, we will ask for accesses.
Thank you for your understanding.
You have to remove the HikaSerial coupon block from your checkout workflow while you're using the non legacy checkout system.
HikaSerial 1.10.4 has been released one year ago ; so that's unfortunately sure that it does not contains the code to handle the new checkout system of HikaShop 3.
Because I'm working as hard as I can on HikaSerial 2.0 ; I'm not able to perform a release right now, but I hope to do it soon.
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.