I'm having the same problem as above and tried the above suggestions to no avail.
It seems an odd coincidence and maybe unconnected but this is what I found in my situation. I have several seperate sites structured the same way. On most of them the Sucuri plugin installed fine. On 2 of them I get the above error and it just happens that on those two sites only, I 'applied' for the API with a non-domain related email ...
I've since changed the email to a domain-related one and tried to reinstall but securi recognizes the site and gives out the same API key.
Anyway, nothing I have tried seems to clear the issue, including removing traces of securi from the DB and reinstalling, to rebuilding the wp-content directory from scratch. Even when copying across the wp-content directory from a working site - the error still persists.
I also noticed I can action all the hardening steps except the one for the uploads directory for which it says is not writable - though is set the same as the directory in my other working sites.
I'd guess it is a owner/user issue related to the API key but would appreciate any help to get the plugin functioning correctly