The Deutschlandticket is valid from the German border so you need a normal ticket for the Czech part of the travel.

Crossing borders and railway tickets is always tricky, and actually on a few stretches the Deutschlandticket is valid in the neighbour countries too up to a certain station, which is specified in this table:

https://www.bahn.de/faq/deutschlandticket-verkehrsmittel-deutschland

Katty's border crossing at Bad Schandau is not in this table, so a ticket needs to be bought for the Czech stretch.

Interestingly, the validity abroad (if any) is typically until the end of a line, but it is not necessarily the first station after the border. For example the Deutschlandticket is valid until Hengelo in the Netherlands, which means that you could commute between the Dutch stations of Hengelo and Oldenzaal, which are both in the Netherlands, with a German rail ticket. 

So there is not a station between Germany and Czech ?

If it's just to use Deutschlandticket from a czech station, these are the three allowed routes (the second one is just possible saturday and sunday).

But of course it's much faster to buy a ticket to the first german station on the direct route to Dresden. 

I want to go go dresden from Prague which station in the nearest 

On the most direct route, the Deutschlandticket is valid from Schöna. But you'd need an additional ticket to get there.

This trip can be done in about 3 hours and 15 minutes. 

But you are aware that you are are just allowed to use regional trains that take 25min longer than the direct EC (most of them with a change)? 

Yea I think it’s better that I buy this ticket thank you anyway 

Only in the Keolis trains though. This was already possible before the Deitschlandticket, using the German regional fares, which are also valid to Hengelo, also only in the Keolis trains. A similar situation exists between Zevenaar and Arnhem.