Currently, recording Tax collected at Source (TCS) deduction in QuickBooks Online is unavailable. With this, I'd suggest consulting an accountant for guidance with other ways to enter the TCS in your account.
You may visit our QuickBooks Blog regularly. So, you'll be updated with the latest news and updates, including product improvements.
Also, please browse through these articles. These links give you information on how the TCS impacts GST and other related topics on managing the QBO account.