Hey @koray1
Have you published your dashboard? I think then only you will be able to see it in that list.
There is an alternative way to do this as well.
Go to your Power Bi Service, then go to my workspaces.

Open your dashboard. After that click on File, then Embed report and then website or portal.

Then a window like this will open:

And copy the url present there.
Now go to your power apps, add a tile control. Go to its tileurl property and paste the url there.

I hope this helps. 🙂