The ground station is not allowed to select this mode

What ArduPilot says:

• GCS entry disabled (FLTMODE_GCSBLOCK)

The ground station (or a companion computer) asked for a mode whose bit is set in FLTMODE_GCSBLOCK, a list of modes that may not be entered by command. The transmitter's mode switch can still select the mode. RTL and Land are not in the list and can never be blocked.

What to do, most likely first

1. The block was set on purpose, to keep a ground station from entering modes that need a pilot on the sticks.

   CheckRead FLTMODE_GCSBLOCK bit by bit: 0 Stabilize, 1 Acro, 2 AltHold, 3 Auto, 4 Guided, 5 Loiter, 6 Circle, 7 Drift, 8 Sport, 9 Flip, 10 AutoTune, 11 PosHold, 12 Brake, 13 Throw, 16 SmartRTL, 17 FlowHold, 18 Follow, 19 ZigZag, 20 SystemID, 22 AUTO RTL, 23 Turtle.

   FixSelect the mode from the transmitter, or clear its bit if the ground station is meant to use it.

Where to do it:Setup & Tuning GuideStarting Parameters