Warning: require(…/wp-includes/compat.php) [function.require]: failed to open

I solved this issue on my blog.

These “Permission denied in” errors are typically the same – chmod is not set properly on PHP files

Using an FTP client (I used FileZilla) is the easiest way and works for any host I have. If you are familiar with shells and have access, just go chmod the file to 755. Otherwse, here is how you do it in FileZilla

1) Open Filezilla and connect via ftp
2) Browse to the directory the file is located (/home/markg87n/public_html/wp-includes/)
3) Right click the file an set the following:
Owner permissions: Read, Write, Execute
Group Permissions: Read, Execute (leave Write unchecked)
Public Permissions: Read, Execute (leave Write unchecked)

At the bottom, the “numeric value” reads 755. If you prefer you can just directly type 755 in the box instead of ticking checkboxes

try it out, and if that doesn’t fix it – you may have to set permissions on more files. Keep refreshing, see the different error messages and plow through it.

Good luck =)