Hi,
The HikaShop SagePay plugin is working correctly on a lot of production websites.
The problem is related to the plugin configuration so I don't think that you have the exact same setup.
See more details about the sagepay error message :
www.sagepay.co.uk/support/error-codes/50...n-missing-or-invalid
If you still have issues ; please contact the SagePay support which would be able to give you more details about what is wrong in your configuration (they have access to logs in their side).
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.