When i armed but do not take off, i can disarm. When armed and take off i can not disarm with APM 2.8; Please help me!
You do not provide enough information. So we can not help you.
What information do you need to help me??
Which Firmware version?
Which vehicle?
How are you arming?
How are you trying to disarm?
Have you followed the tuning procedures? Or did you ignore them?
Provide .bin flight logs.
Why are you using the very outdated and unsupported APM 2.8 hardware?
For the same amount of money you can get a much better one.
My firmware is Copter 3.2.1 and my vehicle is a Hexacopter. I used to calip accelerometer, compass, Esc, radio. I arm and disarm with TX. In my country very difficult to buy a different flight controller 
. I do not have 3dr because cann’t see flight logs. If I update firmware my problem can handle?
You cannot update the firmware beyond 3.2.1. That was the last version that supported that old Flight Controller. None of the Flight Controllers on this list are available to you?
Arducopter
How about this one? Sorry if I guessed wrong about your country from your name.
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