All sleepers and couchettes are already fully booked on Mannheim-Berlin. You can book a seat if you want (not recommended though if you'd like to sleep):

1. Book Paris-Mannheim on www.raileurope.com (desktop website only). Price is €18 per seat.
2. Book Mannheim-Berlin on tickets.oebb.at, using "Seat reservation only”. Price is €3 per seat.

Alternative, you can travel via Brussels:

1. Eurostar Paris Nord 18:25 to Brussel-zuid/Bruxelles-Midi 18:47. Book on www.raileurope.com, price is €32 in 1st class.
2. European Sleeper Brussel-zuid/Bruxelles-Midi 19:22 - Berlin Ostbf 6:27. Book on www.europeansleeper.eu, price is €89 per person.

if I book from Eurail website, I will be getting assured 1st class seats (not sleeper) ? Also once I book from Eurail website, should I go to some other website and do some other payment again ? or all done from Eurail website ?

Yes, you'll have 1st class seats, as shown in your screenshot.

Avoid the Eurail website for reservations if you can: it is almost always more expensive than elsewhere. You can book reservations anywhere you like. For seat reservations in Germany, Eurail charge €8 per person instead of €3 on the ÖBB website.

But if you would like to use a sleeper, do check the alternative via Brussels I posted above.

For general advice on Eurail reservations and where to book, check this page:

https://www.seat61.com/interrail-and-eurail-reservations.htm