Train from Slovenia to Austria

I am not sure what you are asking?

From where to where are you even planning to travel?

Only night trains require reservations in slovenia.

I would like to go from Lesce to Ljubljana without night train and go from Ljubljana to Leoben trough Maribor.

There are some options to do that and all of them do not require a reservation. Also in case you are short on travel days(and not doing both journeys on the same day) Lesce to Ljubjana is a pretty cheap journey (5-7€ depending on the train).

I just noticed that the timetable data for that route is still not completly updated past 14.December (öbb for example is not showing regional slovenian trains for that period)

Also some trains between ljubjana and celje have replacement buses from 14.12 - 30.6