Error When Activating theme

Website breaks when activating Sage 9 after fresh install.

Error: wp-admin/themes.php?activated=true.

My local server errors:

22:13:40 GET /wp-admin/themes.php?activated=true 500 21KB http://mbird.test/wp-admin/themes.php Mac OS 10.13 | Firefox 59.0 mbird.test

I’m using composer to install. When entering the various theme options, I use
http://mbird.test/ as the URL
/wp-content/themes/mockingbird2018 as theme directory

config error on my part. I didn’t install PHP7 on my local image.