If you find trouble to add Superscript in Google Docs at first stage, there are other ways to use Superscript which are as follows;
Select the element which will be Subscript and Superscript then go to Format Tab and click on Text then Select the Subscript and Superscript.
As usual select the element which will be Subscript and Superscript then press CTRL +, (For Subscript) CTRL +. (For Superscript).
To remove subscript press CTRL +, and CTRL +. again.
You have learned in details, how to use Exponents in Google Docs.