Simple answer is yes, under some instances, during OT times. The slavery we know of today however such as stealing and selling people, human trafficking, etc. was in fact outlawed in the Bible ("anyone who kidnaps someone is to be put to death, whether the victim has been sold or is still in the kidnapper’s possession." - Exodus 21:16). Basically you cannot enslave a kidnapped person or their descendants, and basically all slaves are kidnapped at some point ( including the slaves from Africa during the slave trade days).