Try sql('HANA_DB','INSERT INTO MYSCHEMA.ERROR_LOG VALUES([$L_ERROR_NUMBER], {$L_ERROR_CONTEXT} ,{$L_ERROR_MESSAGE} ,{$L_ERROR_TIMESTAMP})');
DS sends the text between the single quotes as such to HANA. That means your global variables are not evaluated. Unless you embed them in square or curly brackets. With the latter embedding quotes are generated,too.
Note that even that statement may fail because of the last column (being a timestamp?). Due to the numerous different internal and external date formats, it is recommended to use a text global variable and convert it explicitly to date according to the format you're using.