Mavlink didn't connect

I have APM 2.6 and I installed Mission Planner.When i want to connect this, there are a problem like no heartbeat packets received.Details are
"Can not establish a connection\n
Please check the following

  1. You have firmware loaded
  2. You have the correct serial port selected
  3. PX4 - You have the microsd card installed
  4. Try different usb port\n\n No Mavlink Heartbeat packets where read from this port
    -Verify Baud Rate and setup
    Mission Planner waits for 2 valid heartbeat packets before connecting

Can you help me?

did you upload a firmware? are the lights doing what they are meant to?