Which exact date?

It´s easier to reserve the Nighttrain vie oebb.at (Add the discount Interrail/Eurail & Click afterwards on Single-/Daytickets and please NOT on the Seat only option )
Here a manual with pictures :)
 

On the 4th July.

I did the way you showed but it didnt’t worked. 

This is how it looks like.

And sorry i forgot but thank you for your help.

And also it is weird for me that in july it is not available (every day until 30 July) and in August it is available every day. Maybe it is because the route changes? So will i be able to book later?

There may be:

1.works along the line so that maybe the train runs a complete different route

2a big school group trying to follow the principles of Gretha from Sweden and rented the whole train/vonat

3. 4th of july-maybe some USAers try to celebrate, but I think its more: SUN, the first when all the Austrians get their holidays=mass exodus
4. Best is to try to use day before/after

Thanks for your answer, but unfortunately it is not available frok 30 June to 30 July (i looked up every single day) so i can’t choose an other day to travel.

So i am thinking about the construction work problem, that maybe it runs on a different route, and i will be able to book later. Do you think this is possible? 

It can be possible
The other reason could as in the time of 30June - 30July the other to Nighttrains to Milan/Rome are not in operation that the Nighttrain to Venice is already sold out :/

Ohh i'am afraid you are right but it is just quite weird that there are available tickets every day instead that period.

Where do you think i can get information about this? 

Call Austrian Rail

Okay, thank you. I'll post the answers so it could help others. 

Seats and couchettes are stil available on Vienna - Venice on 4 July. You most probably selected the "Seat only” option but that does not work for night trains. You need to follow the steps in @seewulf's manual.