Valve had recently implemented a $5 USD minimum purchase in the Steam store in order to accounts to have all features. Predominately, chat features are unavailable to "Limited User Accounts", which is the term used for accounts that haven't fulfilled this minimum purchase. The purchase MUST be through the digital store. Until then, these accounts are unable to send friend invites, put up content on the steam workshop, gain levels for their steam profile and other features.
This comes as a move against phishing, spam and scam accounts that have often plagued the Steam platform.