Are you running this command from the location of the Splunk/bin directory? If you don't have the homepath for splunk configured in your .bash_profile, you would need to run a command similar to this: /"directory where splunk is installed"/bin/splunk enable boot-start
If you don't want to type the full path or cd to that directory, you need to set the homepath similar to this in your .bash_profile:
export SPLUNK_HOME='the directory where splunk is installed'
export PATH ='the directory where the splunk/bin dir is':$PATH
I believe this is a rebootable offense, or at least you have to log out and back in for it to take.