Check the further details in the image you attached: "How to fix it?".
It's very likely related to your .htaccess configurations. You may want to generate it through the AdminCP, can't remember exactly where from the top of my head, but I remember that's what I ended up doing back then and then altering it further as needed. In terms of permissions, IPS is super picky about it; as the error says, 755 for folders and 666 for directories. However, pay attention to this page:
https://invisioncommunity.com/4guides/advanced-options/configuration-options/controlling-file-and-folder-permissions-r279/
IPS writes data to multiple folder and these require 777 depending on your hosting provider. You may want to allow 777 to the following directories: applications, plugins, uploads and datastore.