Both Google products, Google Sheets and Google Data Studio are affected by an issue at Googles account management. (Being logged into multiple Google accounts in your browser at the same time is what can cause this issue.)
A temporary solution before Google releases a fix, is to use Supermetrics in an Incognito-mode window on your browser, and only log in to the tools with the single Google account that has your Supermetrics license.
If you'd like to help speeding up the fix from Googles side, please click on the star in this Google Bug report. More votes means it goes up in their urgency.
We appreciate your help.