error: “cannot update that file/template”

I’m hoping this is the problem; if not, hopefully it’ll help someone who searches for this and doesn’t understand.

Don’t think of chmod numbers as six-hundred-sixty-six… think of them as 6-6-6. It’s a numeric (added) expression of who has permission to do what; it’s read as your permissions, groups permissions, and other (global) permissions. The numbers add up to permissions: 4 to read, 2 to write, 1 to execute — so over 700 may mean you can do anything (7) to the file, but doesn’t necessarily give wordpress permissions to write (2, 6, or 7) or execute (1, 5, or 7). https://www.perlfect.com/articles/chmod.shtml might be of some help, or from a unix shell, type ‘man chmod’ (without the quotes).

Like Iconoplast said, a CHMOD setting of 700 allows only you to read, write, and execute the file in question. This prevents WordPress and your readers from accessing the file. Try either 666 or 755, I use 666.