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Exit code -6 means that the program was terminated by signal 6, which is SIGABRT.

Have a read through https://en.wikipedia.org/wiki/Unix_signal to understand what sorts of signals are possible, and the bash section of https://en.wikipedia.org/wiki/Exit_status#Bash_shell_and_script to understand how those signals become exit codes.

In the context of ROS, this probably means that something called ROS_ABORT(), which isn't very helpful. You should probably look at the rest of the output from move_base to get a better idea of why it decided to abort.