Why can't I transfer music and videos to my iphone on itunes?

It used to work before but now no (heres a screenshot: ) what should I do?

The usual way is to plug in the iphone to your PC or Mac, iTunes should open automatically. Ensure your songs are on iTunes. Click on the iPhone icon in iTunes, click summary and ensure either entire library is checked to sync or selected songs are checked to sync Why can t I transfer music and videos to my iphone on itunes