Im having the same problem with the readiness checkup on magento 2.3.5p1 with php 7.3 (can be changed easily if needed on both CLI and webserver). It gives me a filepath for a subdirectory that doesnt exist For ex: x/public_html/Magento/app/etc/design/frontend/magento
When the documentroot is on the public_html not in the magento folder, this is not defined anywhere so i have no idea where its coming from. Creating the folder manually also doesnt work, permission are correct for other folders, but for some reason it keeps throwing this error