"The man is playing tennis with you outside and with them inside."
Translation:A férfi veletek teniszezik kint és velük teniszezik bent.
I don't think that's why it rejected your translation. I think it's because the sentence is emphasizing that he is playing tennis with you outside and with them inside. So veled has to go in front of teniszezik, for emphasis. The reason veled is being emphasized is because "you" and "them" are the things being compared.
A good way to think of the sentence would be, "It's you who he's playing tennis with outside, and it's they (he is playing with) inside."
Someone named vasvariistvan, who seems to be a native speaker, wrote (near the top of this page) that you never have to repeat the verb "to play tennis" twice. So you're right about that. You might try A férfi veled/veletek teniszezik kint és velük bent next time, to see if that is accepted. (The English doesn't make it clear whether it's you-singular or plural, so you should be able to use either.)
The hungarian present tense actually seems maybe more flexible than the english present simple and present continuous. Hungarian's present tense covers both of english's present tenses, and i beliece can also be used for some future actions where the timeframe and "futureness" are obvious from the context.
Honestly, one of the things i really love about hungarian is it expects you to be able to figure stuff out from context - you dont pluralize nouns if you give an amount, the present tense covers alot of action and you use your eyes to figure out if its continuous, simple, or near future, stuff like that. Its like the language assumes its speakers are smarter than normal.