Looks like it’s trying to use /boot directory. And looks like there is a JIRA for Ambari (implemented in upcoming 1.6.0 release) to not include /boot in the default dir suggestions.
In the meantime, do you want to choose a different dir than /boot on the Customize Services page?
Also, what version of Ambari are you running? The latest is 1.5.1 and info can be found here.