Uh... Not sure, but I take it you're trying to land on a planet?
Here's a quick checklist you need to go through (feel free to add any I forgot!)
1# Are you running ED HORIZONS?
You cannot land without Horizons running. Your launcher should be BLUE.
2# Do you have a planetary approach suite installed on your ship?
If not, they're cheap and have there own slot, and are sold everywhere.
3# Is your planet landable?
If 1&2 are YES, then landable planets will have the blue planet fall icon above them on the Navigation Panel.
4# are you slowing down for your planet?
Hitting them at full speed will cause your FSD to do an emergency drop, slow down to a few 100km/s at the most.