[Shamiul Bishal]

Hello there,
We are glad you are using TOP Table of Contents Pro. It is a pleasure to have you as a customer.

The problem you are experiencing is not actually a problem. When you activate the pro plugin, the customizer option is available on the customizer page.
Appearance>> Customize>> Top Table of Contents

Here is a video that you can check out.
https://youtu.be/6yDPvyD1xDg

It is our pleasure to assist you. Feel free to contact us if you encounter any other problems.

Thanks
Shamiul Bishal