Translation:He doesn't exercise, he only likes to sleep.
"He does not do exercises; he only likes to sleep." is rejected.
It looks like "exercise" ("運動") is being used as a noun here, so singular or plural shouldn't matter.
I feel like," He doesn't like to exercise, he only likes to sleep." should be accepted as well. ;-;
He doesn't exercise, he only likes sleep. This should also be acceptable?
Nope. Neither is the answer given by Duolingo. It's a run-on sentence in English. You should use either a semicolon or a fullstop instead of a comma. Alternatively you could use conjunctions like 'and'.
Can this mean 'He doesn't like playing sports, only likes sleeping'? 运动 was used to mean sports in a previous exercise.