Could you please click "Show" to display detailed error message? What version of Power BI gateway do you use?
Also please share gateway log here, you can export gateway log following the guide in this article.
I am Sorry but i am not admin so i am not able to Extract the Log but please Find the More Details box attached.
Also for the Gateway, i am using the on premisis not the personal one.
FYI, this is a SSAS Connection.
Could you please remove the SSAS data source from gateway and check if you are able to add it again?