New to this , Ive had Photoshop elements for a few years now on my macbook air completely problem free , but recently its started to play up , when you have the image for editing its ok then as you adjust the levels for instance the image disappears untill you stop moving the slider when it reappears , this happens on all adjustments (sharpness,lighting,colour etc)
The problems you are getting with Adobe Photoshop Elements 15 are a result of you upgrading your MacBook Air to macOS 11 Big Sur.
Unfortunately, Photoshop Elements 15 is nearly 5 years old now and was only tested on (and thus is only officially supported on) OS X 10.10 and 10.11. It still does work on later versions of macOS, but Big Sur is not one of them.
If Photoshop Elements 15 is one of your main day-to-day applications I would highly suggest downgrading your MacBook Air back to your previous version of the macOS.
Otherwise, you will need to upgrade to Adobe Photoshop Elements 2021. If you purchase it via direct download from Adobe you will be eligible for an upgrade discount as you are an existing Elements owner.