The BETAFPV BNF drones use the switch on chanel 5 (AUX1) to arm. Here is the default configure on Betaflight GUI. This article is also effective for BETAFPV flight controllers.

First, please make sure the radio transmitter has bound to the FC correctly. Please check "How to bind the radio transmitter" article for help.

Just plug the FC into Betaflight GUI and go to the Receiver tab, the board should respond to radio transmitter commands when you move the sticks on your radio transmitter. As show below.

Here are some usually situations that can not arm with throttle.
1) Keep the quad horizonly when power up.
2) Make sure your receiver end-points are calibrated to go full 1000-2000 and center at 1500. It might be expecting throttle to below a threshold before arming. Make sure the the Stick Low Threshold is high than the mininum value on Throttle. As show below.

3) For BETAFPV F3 EVO flight controller board, because of the "swamping" problem, please keep your radio transimitter 1 meter a way from the drone when arming.

Most Common Reason

The flight controller is not assembled levelly on the frame. As a result, the drone could not finish gyro calibration and can not arm. 

Here is the video by Joshua Bardwell about the possible causes that could be preventing your quadcopter from arming. You can find the one that's affecting you, fix it, and get into the air!