Sure you didn’t accidentally activated a travel day in the UK (your home country)? Can you check all your planned journeys?
For example: if you travel down from your home to London on day 0 by train and pass, and then take the Eurostar to the continent on day 1, you will have used up all your travel days for the return journey.
Here’s a tutorial on how to use your mobile pass: https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started
It seems complicated at first, but once you’ve got the hang of it, it’s pretty straightforward. The most important thing to remember is only to activate a travel day, until just before boarding your first train of the day (just minutes before) and the limitation of two travel days in your home country.
I don’t think it’s possible to change a mobile pass to a paper pass on such short notice, printing, shipping,… takes a while and it won’t be there before you leave I fear.
The message that comes up is outbound inbound journey used up. I've written to InterRail themselves but of yet to receive a spot response but we'll take it further with them. I looked into it a bit and it appears that you're only able to take one output outward and one inbound journey to your country of residence.
This wasn't anywhere in the advertising and so I booked Eurostar to Belgium and return to London and then I went on to the interail site and was able to reserve tickets again from London to Paris and from Paris to London.
Places where the complications seemed to have a risen.
I definitely my travel days have been perfect so far I'm only glad that I looked at my travel day a week before I traveled, as there is clearly a big glitch with traveling from the same country more than once.
I’m afraid that this rule is very clear and is not in any way hidden.
https://www.interrail.eu/en/interrail-passes/global-pass
How it works
How long is a travel day?
A travel day ends at 23:59 CET of the day you travel, on which you can take all the trains you want! Curious about night trains or time zones?
Can I travel in my country?
Only to cross the border. The Global Pass allows up to 2 journeys in your home country: 1 to leave, 1 to return - multiple train transfers included.
Since you won't get to Paris by Eurostar (unless you buy an expensive ticket) here are some alternatives: https://www.seat61.com/trains-and-routes/london-to-paris-by-ferry.htm
Newhaven is the easiest. Or there's always Flixbus or flights in case...