cannot access dashboard of newly added multisite subdirectory

Hello, I added the site following the “add site” directions and, although the new site shows on the list of sites I cannot access the dashboard.
I looked through similar questions, and some of the responses reference updating the .htaccess file. I’m not sure what this means. Please advise.

My primary site is https://serendology.com and the new subdirectory is design portfolio.

Thank you!