Apps not installing on my Apple Watch

If you followed the steps in the 'Install apps you already have on iPhone' section of:

Get more apps on Apple Watch - Apple Support

then the first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

Also, since your tag sys you are using watchOS 4,. see the replies to:

Apple Watch 0 not installing apps - Apple Community

"Because the apps are now created to run on Watch OS 7, they are incompatible for OS 4.3.2, so it is now impossible for them to load on our Series 0 apple watches."

Apps that are not updated to run on the latest hardware and software are removed from the appstore,I assume this is the same for watch apps.

Have you tried installing one of the apps from the app store?.

Your watch is too old to run more demanding software and your watch's hardware can't manage it.