I have available storage, but can’t install apps For a while now I’ve been having trouble installing apps on my iPhone. Even a small app (50-100 mb) can’t be installed. It says that the device has no available storage left to install this application. I can’t even install a firmware update so I’m stuck at iOS 12.1.4
If you have any partial downloaded app, large file etc which has not been removed, this might can occurred. Clear Cache or delete partial downloaded large file and that might have cause your problem.
A quick glance of how to clean the cache in iPhone.

-Tap Settings > General > iPhone Storage. (On an iPad that option will be labelled iPad Storage.)

-At the bottom of the screen you'll see your apps, arranged by the amount of storage they take up. 

-Tap an app that's taking up a lot of space(In this case application download data and store them in a location of the iPhone

-Take a look at the entry for Documents &amp; Data. If it's using more than 500MB, it's worth deleting and reinstalling the app to clear the space.
Tap Delete App, 

-confirm