I am making an app, which senses if you logged in and plays my music until the end, but if you didn't, play just 30 seconds. I will show you the error pages.
It looks like these are screenshots from 2 different screens? Which ones?
Use a boolean true
To store information persistently use TinyDB
And fix the red error on the other screen
See also tip 1 here General Tips and Tricks for App Inventor
A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook App Inventor 2 Book: Create Your Own Android Apps ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
There is a free programming course here Course In A Box and the aia files for the projects in the book are here: App Inventor 2 Book: Create Your Own Android Apps
How to do a lot of basic things with App Inventor are described here: How do you...? .
Also do the tutorials Our Tutorials! to learn the basics of App Inventor, then try something and follow the Top 5 Tips: How to learn App Inventor
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
The first one is the select page. You select weather to log in or not. And the second one is the music page.
So i can't use the loginbolean in the 2nd screen.
Right, global variables are available only in their own Screens.
But TinyDB tags/values are good places to store values that can be accessed in other screens.
can I use serverstorage instead?
You can, but it would mean an unnecessary round trip to your server, when you can easily use tinydb to do the same thing.
how can I use TinyDB to store this boolean?