Hi @Susan Barnes ,
Under normal circumstances, there is no need to do some settings, the data should appear automatically no matter it is classic or modern.
It shows that the data is not available on one of my sites:
Data(average time spent) can be displayed on another site, however when using time spent sorting, there is not enough data:
Per my knowledge, this average time spent feature was added last year, and there is currently no rich information or posts.
I would suggest you open a service request for direct help:
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