/admin/config/development/performance then the site will be trying to aggregate css and js files below the
sites/default/files directory. However that site is not included in the codebase cloned from Pantheon, and you might not have thought to create it, especially if you haven't added files to your site.
/admin/config/development/performance, then refresh the page. If that doesn't work then click "Clear all caches" on the same page. However your site will perform better with those 2 aggregation boxes checked. So...
A better solution is to visit your site's Pantheon dashboard, switch to the Git mode, open "Backups", create a new backup, then download the "Files" backup. Extract that downloaded tar.gz file which will give you a directory called something like
files_dev. Move that directory into
sites/default/ and rename it to
files. Check that it has
drwxr-xr-x permissions. If not then
cd to its parent
default directory and issue the command
chmod 755 files.
You should also check that your
sites/default directory has
drwxr-xr-x permissions. If not then open a terminal,
cd to your
sites directory and issue the command
chmod 755 default.
Refresh the site and if necessary clear all caches and the css/js should work again.
For me this fixed a similar issue after importing a site from Pantheon to Lando.