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

If you do a google search using the error message 'Page not cached by WP Super Cache. No closing HTML tag' you will see this has occured many times over the years with WP Super Cache.

My suggestion would be to try another caching plugin.