The QR code changes every 60 seconds. Depending on your internet speed, you might not have enough time to submit your attandence if you scan it too late. You should try to scan the new code as early as possible or establish a faster internet connection.
If you are using an iPhone, changing your phone's default browser to Chrome and accessing LMSCLUS through Chrome may resolve the issue.
Before scanning the code, you should have logged into your Microsoft account on your phone with an email address ending in ostimteknik.edu.tr.
If you have other Microsoft account(s) different from email address ending with ostimteknik.edu.tr, make sure you are connected to the Microsoft account with the ostimteknik address on your phone, not the other(s).
You may accidentally believe that you could not mark your attendance with the QR code, although you could in fact. Thus, please ask the instructor of the course to check your attendance.