I am trying to set enumerated value to field in a session as follows
TsNfunctionCallVoid = stpapi.enum.answer("tdsls4401M000","selection",2)
** Reply GetObject send for process 3, object 78
attribute DsNdllName : (char *)(13)="ottstpapihand"
attribute DsNfunctionCallVoid : (void *)(50)="stpapi.enum.answer("tdsls4401M000","selection",2)"
** Request CreateProcess received for process 4,object 0
attribute DsNname : (char *)(14)="ttdsls4401M000"

and receiving this error
attribute DsNstring : (char *)(45)="Can't read session or object (tdsls4401M000)"
** Request DestroyProcess received for process 4,object 0

What am I doing wrong ?

Try changing "ttdsls4401M000" to "ttdsls4401m000", so with a lowercase "m".

Thanks Marnix.
Yes, that was the problem