Hello. I guess you need to check the timetables separatedly:

From Riga To Valga here

https://www.pv.lv/en/routes/?from=R%C4%ABga&stop_from=1&to=Valga&stop_to=&date=24.06.2021&date-h=24.06.2021&transfer=&stop_transfer=

and from Valga to Tartu here:

https://elron.pilet.ee/en/otsing/Valga/Tartu/2021-06-24

Many thanks ! 

Do you know if it’s really necessary to change in Daugavpils to travel between Vilnius and Riga ?  

And I din’t find ticket to train for Bialystok-> Kaunas and Vilnuis-> Daugavpils on https://www.traukiniobilietas.lt/portal/tickets?departure=2400440&departureDate=2021-07-15&departureTime=00:00&arrival=2400000&returnDate=2021-07-15&returnTime=00:00&routeDirectionType=FORTH, do you know how it’s possible to do the reservation or see the schedule ? 

Thanks by advance. 

Yes, unfortunately. Also see:

https://www.seat61.com/trains-and-routes/vilnius-to-riga-by-train.htm

If the train Kiev - Minsk - Vilnius - Riga starts again, then there will be a direct possibility again (every few days).

I'm not sure if international trains from/to Lithuania already run. The Lithuanian train planner doesn't show them indeed and neither does the Polish train planner. The best is to ask Lithuanian railways when they will start running again: informacija@litrail.lt

In any case, you don't need any reservations for Bialystok to Kaunas, Kaunas to Vilnius or Vilnius to Daugavpils. You should even consider if it's worth using a travel day because these trains are quite cheap. For example, Vilnius - Daugavpils is 9 EUR; Białystok - Kaunas is 11 EUR.