'Failed to connect to master in 30 seconds.' error

Hi, All! I am trying to run Gazebo on Ubuntu.

make px4_sitl gazebo_(my model name)

When I run the above code, I get the following error:

[Msg] Waiting for master.

[Err] [ConnectionManager.cc:121] Failed to connect to master in 30 seconds.

[Err] [gazebo_shared.cc:78] Unable to initialize transport.

[Err] [gazebo_client.cc:56] Unable to setup Gazebo

If anyone knows about this error please leave an answer.
If you need more files, I can give them to you. Thank you!