The updated or inserted row is referenced using the new record:
CREATE OR REPLACE FUNCTION trigger_fct_tri_sv_tab_temp()
new.FLAT := UPPER(new.FLAT);
There are 4x different RDBMS products under the Sybase name ... Adaptiver Server Enterprise (ASE), SQLAnywhere, IQ and Advantage. The 4x products do not share a common SQL language/syntax.
The question references a link for the create trigger syntax in ASE.
The sample code in the question appears to also be for ASE.
However, from the comments we've ...