Q: Not able to uncheck/deselect any songs from iTunes

Hello Balagct22,

In what aspect are you having issues with unchecking items in iTunes, are you referring to syncing? If so, you do have the option to manually drag and drop the songs to your iPhone. Also, you might want to try starting up iTunes in Safe Mode to test if you can uncheck items in iTunes. Take a look at the article below for more details. 

iTunes 12 for Windows: Sync iPod, iPhone, or iPad
https://support.apple.com/kb/PH20343

Fix unexpected quits or launch issues in iTunes for Windows
https://support.apple.com/en-us/HT203206​

Take care