Why am I getting "maximum number of users" error in MATLAB?

When I try to run MATLAB, I get the following error:

License Manager Error -4.
Maximum number of users for Image_Toolbox reached.
Try again later.

This indicates that all of the licenses available to you were already in use by other people. Often, people leave MATLAB processes running without realizing it. In some cases, people can log out from a machine, but inadvertently leave a MATLAB process running in background.

The lmstat utility will show the list of MATLAB processes that are currently holding licenses.

You can purchase additional seats for a floating network license.