You are probably using the Applications Manager in the Linux machine in the headless mode. The DISPLAY variable has not been set properly.
Solution 1: Do the following steps to set the DISPLAY variable before starting the Applications Manager:
1. Export DISPLAY=machinex:0.0 [This should be executed in the Applications Manager machine to set the DISPLAY variable]
2. xhost + [This should be executed in 'machine x' to provide permission for the remote hosts to open the GUI] where "machine x" should be a Linux machine with XWindows installed (with Display).
Solution 2: Execute <unset DISPLAY> command and then execute the Applications Manager startup script.