Hi IRHM73,
Try to change your query like follow because have alrady prefixed author in dropdown input:
<query>|rest /services/search/jobs |search "$author$"
|search NOT (author="splunk-system-user" OR author="monitoring")
|search title!=""
|addtotals fieldname=duration *duration_secs
|eval groupduration=case(duration<=300, "Less Than 5 Minutes", duration>300 AND duration<=600, "Between 5 and 10 Minutes", duration>600 AND duration<=1200, "Between 10 and 20 Minutes", duration>1200 AND duration<=1800, "Between 20 and 30 Minutes", duration>1800, "Greater than 30 Minutes")
|convert rmunit(duration) as numSecs .......
....
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