Everything seems to be correct, and yet the value is not generated.
Most of the time the problem lies in customizing:
Check or maintain the generation class for the object type. If no generation class is entered, no generation takes place.
Check or maintain the technical type (TECHTYP) on the element (TME, PMK...). The signature of the generation method must match the technical type.
Maintenance type (FILLTYP) on the element or element assignment.:
for A, the value cannot be entered manually.
at M nothing is generated, (only manual or automatic input)
R offers the greatest flexibility: the value can be generated, and can also be overwritten manually.
Format of the element (TYPE) Min/Max/Nom etc.: only the corresponding columns are displayed/populated.
Priority flag for value determination (PRIOGENREF): With the help of the BADI /SCT/QP_BADI_OBJTYP two competing methods are available for selection:
(G) The value is calculated via a generation method.
(R) The value is determined from a reference.
The default value is R: generation has priority over referencing.
If the desired line should not appear after the generation has run, it should be checked whether the line is visible. If necessary, the VISIBLE attribute in the generation method should be set to abap_true if the element was not set to "Show initial" in Customizing.
the value was adjusted manually before: Normally, manually entered data is not overwritten with a generation, unless "Generate with overwrite" was triggered.