The secure element in the device is limited in memory space and only 4-5 apps (Nano S) or 8 apps (Blue) can be installed simultaneously.
If you want to use more simply uninstall temporarily one app and replace it with another.
All apps can be installed/uninstalled without any risk to lose your funds. You can uninstall all apps an re-install them to reset the whole memory fragmentation.
To uninstall an application, launch your Ledger Manager.