Tummy sleeping for a baby is okay after the baby turns 1 year old since by then they'll usually be able to turn themselves back over.
When the baby is under 1 year old and they're put on their tummy then they won't be able to turn back over and that does increase the risk of SIDS.
So babies should never be put on their sides or tummy for sleeping until they're at least 1 year old or older.