A lightning network channel has a certain transactional capacity, determined by the channel participants at the time of creation. While it is possible for both parties to add funds to a channel during the creation of said channel, the current implementation generally has just one participant commit funds to the channel. So generally, a channel will start with all of the capacity on one end of the channel, and no capacity on the other end (since just one participant funds it).
To illustrate this, imagine you open a channel with 1BTC capacity to your friend. The initial channel state is:
So if your friend wants to pay you, they will need the capacity to do so. Maybe they open a channel of their own to do this, or you send some transaction that pushed BTC to their end of the channel, or maybe in the future the channel will just be dual-funded in the first place.
Whatever the case, just because you are a participant of a channel, that alone doesn’t mean you can send/receive lightning payments. Rather, you need to be a participant of a channel, and have the appropriate channel capacity available to your node. This works both ways: you can’t get paid if all of your channels are 100% pushed to your end, and you can’t send payments if all of your channels are 100% pushed to your friend’s node(s).