Hello Joseph,
I assume you are using a HANA revision < SPS10. To my knowledge in revisions before SPS10 it is not allowed to change transition variables in BEFORE triggers. Or said in other words. The first time this "feature" is explicitly mentioned is in the CREATE TRIGGER documentation for SPS10 (CREATE TRIGGER - SAP HANA SQL and System Views Reference - SAP Library).
As alternative you can use the sequence directly in your INSERT statement. But of course you have to do that at all places where you execute the INSERT statement (I think with the trigger you would create a single place where the sequence value is determined; if not the need for the procedure is not clear to me).
Best Regards,
Florian