I began learning using Esperanto in 12 Days.
I had to translate this : "What is on the work-bench in our room?"
What I initially thought : Kio estas sur la laborotablo en nia ĉambro.
real answer : Kio estas sur la labortablo en nia ĉambro.
Why doesn't labor take an -o at the end? Basing myself on the text example of the webpage, it seems like compound words (even if they are are verbs) take an -o.
I'm not familiar with the site Esperanto.com and can't speak for its general reliability. Airvian is correct that the O is optional in compounds and the decision whether to include it usually comes down to how easy it is to pronounce without it. In this case, you probably would leave it out. That is labortablo is more common.
In turn, stablo is more common than labortablo - but the course rejects this answer.
In word compounds one can add a vowel in the middle to make the pronunciation easier, but it's always optional. But that means that labor·tabl·o and labor·o·tabl·o are equally correct.
For more on that I highly recommend the article in English “Word formation in Esperanto”.
The -o is usually omitted unless its presence eases pronunciation (in the opinion of the speaker).
Source: http://pages.ucsd.edu/~dkjordan/eo/colloq/colloq040.html#sec4-1-2
