groovy.lang.MissingPropertyException: No such property: manager for class: Script1

manager is provided by certain Groovy script plugins, but not all. To make your script generic, use the Jenkins/Hudson API instead:

import hudson.model.*

def build = Thread.currentThread().executable
def buildNumber = build.number
...

Just in case it helps, if you are using the 'Execute System Groovy Script', you don't need to use the 'manager' variable. This worked for me -

def workspace = build.getEnvVars()["WORKSPACE"]

One of the reasons groovy.lang.MissingPropertyException: is thrown when you are using a variable outside of its scope or you haven't defined that variable.