That's because there is no train from Helsinki to Tallinn, but a ferry. 

I know, but I meant there is no “Tallinn” in the list of cities. I read only “We couldn’t find any stations matching Tallinn in our timetable”

I guess that means that there are no routes to/from Tallinn in the planner.

You can always add a journey manually to your trip if you travel to/from Tallinn.

According to the info on interrail.eu you need to make the reservation in Estonia. That would explains why you can't see Tallinn in planner.

https://www.interrail.eu/en/plan-your-trip/reservations/how-do-i-book-my-reservations

EE=Estonia has only 1-and quite bad- train connection to LV=Latvia via the ugly and nothing to do border town of Valga. NO trains at all for now between LV and LT=Lietuva=Lithuania and ditto LT-PL=POland.

There are 3 or 4 ferries doing the HEL-TLN run, passholders may get a little discount on some.

A pass is often reported to only bring misery and disappointment in these 3 baltic countires-and is anyway far more costly as paying local tickets

Trains in EE are NON_reserved-in fact there is not even except TLN a station anymore that will sell tix-the conductress does that-to the very few that do not have a local chipcard

interrail.eu says it is not possible in 2nd class.