1. Do you have a mobile or a paper pass?
2. If it's a mobile pass, you using a number that consists of a capital i and 8 digits? They usually want that format. You can generate such a pass cover number from the mobile pass number here.
3. Is this the first ferry booking you're making?
4. If not, are you booking the ferries in chronological order? That is required for some reason.

i have a paper pass, and I have already booked the 2 International travels included in the pass, and now I need to book the domestic ones, and I have troubles with them.

OK, could you then please look at question 4?

I booked at the same time the international travels from Italy to Greece and from Greece to Italy, and then the intention was to book the first domestic travel (from piraeus to Mykonos), which should be the first national travel of the 4 we can use with the pass, in chronological order.

Maybe you can book via Blue Star ferries? https://www.bluestarferries.com/en-gb

If that doesn't work either, then contact customer service here:

https://www.hellenicseaways.gr/en-gb/contact

yes I tried also via Blue Star Ferries, but I think the problem could be the fact that I didn’t received the pass yet, so I think the number that I'm using could probably be wrong, because I'm using the number they gave me at the time I bought the pass. I saw from other messages in the community that also other people have had the same problem and they told them they would have give them a provisional cover pass number, to be able to book travels even if they didn’t received the paper pass yet, in order to guarantee a seat reservation