The resolution of my monitor is not available

macOS might hide by default some resolutions depending on the monitor and the connection type (HDMI or DisplayPort for example).

To show additional resolutions:
- Open the Displays system preference 
- Find the preferences window for the screen you want to adjust
- If not already selected, choose the "Default for display" resolution setting
- While keeping the option (alt) key pressed, choose the "Scaled" resolution setting

The complete list of resolutions accepted by macOS should now be visible.

If you have a 4K display and text and windows are too small for you, you can choose the "1920x1080" mode from the list. This will set the display to the 4K retina resolution.