Where are you trying to book?

Additionally, on which date and from where to where?

With Eurostar you have to use Standard Premier. Business Premier (same seats and layout as Standard Premier, have extra meals) is not possible with Interrail. Cost: 38€ or 43€ to Amsterdam. 

2nd class is Standard. (30€ or 35€ to Amsterdam)

You can buy reservations for Eurostar also via:

 Eurostar High-Speed Train | Chunnel Train- Interrail.eu

How to make your reservations for the Eurostar 

You can reserve your seat on the Eurostar in the following ways:

• Through the Reservation Service.
• Through the Rail Planner App.
• Through the SNCB International website (paper pass cover number needed)
• Directly from the Eurostar terminals.
• By calling the Eurostar booking center: +44 1233 617 575 (outside the UK) or 03432 186 186 (within the UK)
• SBB Stations in Switzerland
• Railtourguide (UK based travel agent)

Trying to book from London - Paris July 31st

So have I made a mistake purchasing a 1st Class Interrail pass if I can’t use it in Premier on the Eurostar?!!!

What is the point in having a 1st class pass if I can’t use it?

I see first class available on all but 1 Eurostars:

(Also some later options with 1st class available.) Same results on interrail.eu.

This is 1st class (wider seats, more legroom etc.), just that you only get a "light meal”:

https://www.eurostar.com/be-en/travel-info/travel-planning/travel-classes/standard-premier

I am trying to book 1st Class ticket to Paris from London on the Interrail Website with my 1st Class interrail pass - no i=option for 1st class travel with my first class pass??????!!!

The train arriving at 13:47 is currently the only train that doesn't have pass holder seats anymore in 1st class (the same train shown in my screenshot). You can either book this one in 2nd class or another train in 1st class.

The 1st class on Eurostar is the Standard Premier you will get even a small meal and get the same seats as in Business Premier (they have Lounge Access, shorter Check-In limits)