You have installed the stand-alone version of Lightroom 5.7. In order to activate that you will have to have a valid serial number for the stand-alone version. The application manager verifies everything by Adobe ID and password. It's difficult to mix creative cloud and standalone versions. You would probably be better off to uninstall the stand-alone version and try to install the update through the creative cloud manager again.
If I remove 5.7 will I lose all my Lightroom setups ??
Everything that you have done in Lightroom is stored in the catalog, which is a user file. User files are not removed when Lightroom is uninstalled. After you install from the application manager you should just have to double-click on the program icon to start where you left off. At least that was my experience.
You should have updated via the Adobe Creative Cloud Application Manager since you are "on monthly subscription".
Uninstall the downloaded 5.7 application this procedure will not affect your Catalog, Preferences or personal files.
Shut down your computer and then reboot.
Open the Adobe CC Application Manager and login it should show the correct LR download to install. There will be no need to manually input a serial number.