r - Getting "(Type Error): null is not an object (evaluating a.length)" in RStudio -


i used readxl library read in excel file r studio , i'm trying access column in dataset using $ operator. however, keep getting "(type error): null not object (evaluating a.length)" notification though i've performed type of operation many times before without issue...

the error i'm getting:

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the dataset in global environment pane:

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