Some people maybe meet this problem using APM flight controller.
How to solve it?
APM cannot use “Install Firmware”.Should use “Install Firmware Legacy”.Because “Install Firmware” is used for PIXHAWK flight controller.
So use “Install Firmware Legacy” to upload firmware.
Select “install Firmware Legacy”, do not select “Install Firmware”,because “Install Firmware” is used for PIXHAWK,not APM.
Make sure you can see the Version number,if you can not see the version number,it means that your mission planner cannot connect 3DR firmware service, you can not install firmware online.
4. Do not upload latest firmware (for example 4.0.3) offline for apm board.
Due to the limited memory space of the APM board, the latest firmware cannot be uploaded. If you use the mission planner to flash online, it will automatically help you to upload the firmware suitable for the APM board.For example, the copter firmware for apm is 3.2.1 (mission planner will help you upload this version),the latest version is for PIXHAWK , if you upload offline above 3.2.1, it will clear the bootloader of chip, when bootloader is broken, apm cannot work, also cannot upload new firmware.
APM 2.x (APM 2.6 and later) are no longer supported for Copter, Plane or Rover. The last firmware builds that fit on this board are Copter 3.2.1, and Plane 3.3.0, and Rover 2.5.1.
So, upload firmware online at mission planner, if you upload offline, make sure the version is not above 3.2.1(copter, for example)