I am trying to plot DAU/MAU in google data studio but when I try to create a formula it always says invalid formula.
Additionally, if I try to insert a scorecard with DAU it always fails.
Please advice.
I am afraid you may be coming up against the fact that GA data does not present to GDS as a single data set with all columns in, but rather as different purpose subsets which can't all be joined together. What you could do (as recommended by my son who does this stuff on a daily basis) would be to use the GA plugin in Google Sheets, use formulas there to get the data as you want it, and then point GDS at your Google Sheet data.