There might be several reasons.
1. Make sure that you’ve chosen the appropriate account type in the right panel in your Members Area.
Any MT4-based account can be used for subscribing to traders except for "CopyFX Trader" and affiliate accounts. You can also use MT5 accounts with with a hedging model.
2. Make surу that you have sufficient Equity on your account to connect to a Trader.
The Equity value can’t be less than the minimum deposit specified by the Trader.
3. Make sure that the Trader accepted your subscription request.
If you subscribe to the Trader with "By request" subscription mode, they have an opportunity to reject your request or consider it whenever convenient to them. Check your email box: if there is no message with rejection then your request is still under consideration.