Q: Can't change lockscreen wallpaper

Hi, RonRebel.  

Thank you for visiting Apple Support Communities.  

I understand that you are unable to set a new Lock screen wallpaper.  Let's start with forcing all open applications to close and restart the device.  Once this is done test setting a new wallpaper.  

Force an app to close in iOS

If this does not resolve the issue, test setting the Lock screen wallpaper with a different image.  

Change your iPhone wallpaper

Cheers