Thanks @David Browne,

Thank you for the first tip!

But the issue is I was trying to create a column and your query gives a table context. But thankfully I finally was able to create the below query that works based on the answer you provided! Thank you so much!

CustomerRevenueTable = 
SUMMARIZE(
    '''Online Retail2$''',
    '''Online Retail2$'''[CustomerID],
    "Total Revenue", SUM('''Online Retail2$'''[REVENUE])
)