A given username can only have one trading session active at a given time. If you have a trading session running (e.g. TWS or IBKR Mobile) and you start another session (e.g. Client Portal), you will be warned upon login to Client Portal with the message:
"You are currently logged in without Trading/Market Data permissions. You are logged into the Brokerage system on another platform. You can only trade and view real-time market data on one platform at a time. You can still manage your account but cannot trade. Another session with the same user name has disconnected your session."
To enable trading access on Client Portal and disconnect the other trading session, click on the "Log in with Trading/Market Data" button.