That train is fully booked. No reservations available, not even on the mandatory part Padborg-Flensburg.

Travel 2 hours later with a change in Flensburg, no reservations needed.

Don't book through interrail.eu as optional seat reservations are more expensive and there are extra fees.

For the cross-border trains with mandatory reservations : cd.cz 73 CZK (3.10€)

For trains from/to/within Germany : bahn.com 4.90€ per journey

For trains in Germany and many other countries : tickets.oebb.at 3€ per train (free with a 1st class pass)

Where I should book for this train with EU global pass? I have already check with https://www.bahn.com/en still have a seat.

Well it's explained on my post. You won't be able to take the 08:24 train as it's fully booked.

You need to go further into the booking process to see that it is fully booked.