Home location must be set correctly. I usually right click on the map and select "set home location". You must supply the altitude in meters at that place if you do it that way. If you get the altitude wrong, then your waypoints (relative to home) will be wrong. That may be why it seems to land when you engage PathPlanner.
There is another, better way to set Home Location if it is easy to put the model at Home Location: With model powered, connected via telemetry with a good GPS lock, and model sitting where you want the Home Location to be, in GCS go to Configuration->Attitude->Settings. First press Clear so the coming change is obvious. Uncheck "Is Set" (I don't think you need to press Save here to see the change to Is Set, but I might be wrong) and within a few seconds it will fill in the current UAV location into the Home Location. Press Save to save the new Home Location. Because I don't do this very often, I would power the UAV off and on and verify that the Home Location is stored correctly.
With model ready to fly, powered on, connected via telemetry:
I just click a place on the map where I want to go, then press Ctrl-A to add the waypoint. You can also right click the location and select "add waypoint". Repeat for all the waypoints I want.
Then I bring up the waypoint editor and add 3 for altitude for all waypoints and 2 for speed for all waypoints.
I usually save it to disk, but that is not needed.
I then send it to UAV.
I then take off in Attitude mode and switch to PathPlanner mode.
Be aware that the default is that it will loop to fly the set of waypoints over and over.