There is no availability from Paris to Amsterdam on Monday 7th August. Thalys has a quota for passholders and it can sell out well in advance.
Alternatives :
- Thalys to Brussels (27€, multiple departures available) + hourly IC Direct to Amsterdam (reservation-free)
- TGV to Lille (10-20€*) + reservation-free ICs via Kortrijk and Antwerp
*As you live in France, you can get free TGV seat reservations by going to a SNCF ticket counter. Ask for a billet parcours (ou trajet) d'approche
Do not hesitate for questions.
Btw there's no need to activate the pass in order to buy reservations. You can buy them on the website or via the railway companies (see the link above). It's even best not to acrivate it in advance in case.