• Venezia S. Lucia to München Hbf (09:10 → 16:35 on 13 July 2022): Reservation optional for this train, but recommended in summer, weekends and holiday season. But you need to pay 10€ (€=Euro) fee (2nd class, 15€ 1st) for the italian part of the trip. Online you can book via tickets.oebb.at - one way ticket option and at adult choose discount Interrail/Eurail Global Pass. Reservation costs 3€ same website but you choose the seat only option. At Ticket Desks of Trenitalia in Italy you get with buying the fee a reservation for free. 
• München Hbf → Prague Hlavni Nadrazi (10:43 → 16:21 on 16 July 2022). This trains has only optional reservation. I’m not even sure if it is possible to reserve a seat, because it runs as regional train in Germany. If it is possible try via seat only option of Czech Railways cd.cz  should cost about 3€ (the website will show you Czech Krone) 
• Berlin Hbf → Amsterdam Centraal (09:07 → 16:50 on 23 July 2022), this train has optional reservation. You can book the reservation directly via seat only option of bahn.com (4,50€/journey 2nd class or 1st class 5,90€/journey) or via Austrian Railways with the seat only option at tickets.oebb.at for 3€/train. 
• Amsterdam Centraal → Paris Nord (09:15 → 12:35 on July 26 2022) Online you can book it via Eurail.com, but you tried this. If you have a paper pass you can book online via B-Europe (you need a paper pass cover number. You can request one via customer service if you have a mobile pass, but they are overrun by questions currently) you can also call B-Europe Call Center. Or at Ticket Offices of NS, SNCB/NMBS and SNCF. You can check without cover number if the train you want to take is already book out for pass users via B-Europe. 

München Hbf → Prague Hlavni Nadrazi can be booked via DB (€4.50), ÖBB (€3) and CD (CZK 76) but only 30 days in advance.