I have Atahualpa 3.7.24 installed and just installed WP Super Cache. Enabled caching and tried the test but caching is not working. Inspected my homepage and see this error:

<!-- Page not cached by WP Super Cache. No closing HTML tag. Check your theme. -->

Do I have a corrupted theme file or is there some other issue?

Thanks for the help.

debugging 101 - disable ALL other plugins to see if there is a conflict - you may allso want to swap to a twenty-xxx theme and test to see if the same problem occurs

I also notice that you opened an issue on WordPress and say that when you look at the page it shows the </html>.

I just tested with the same versions of WP, Atahualpa and WP Super cache and have no problems

I switched from Atahualpa to Twenty Fifteen and caching worked flawlessly. Appears to be a theme issue. Could it be a combination of Atahualpa and a plugin?

I seem to have found the conflict - Atahualpa and Next Gen Gallery. Get this error:

<!-- Page not cached by WP Super Cache. Blank Page. Check output buffer usage by plugins. -->
<!-- Page not cached by WP Super Cache. No closing HTML tag. Check your theme. -->

Once again, I have tested the same versions and I have no problem.

What is the url?
If you disable caching and look at the source of the page, is the </html> there?

spswoodturners.org

Seeing the closing html regardless of plugin status.

Caching disabled, nextgen active:
http://www.spswoodturners.org/debug/...en-active.html

Caching on, nextgen active:
http://www.spswoodturners.org/debug/...en-active.html

Caching on, nextgen deactivated:
http://www.spswoodturners.org/debug/...deactived.html