The message, "It seems the product key you entered contains unrecognized characters. Please correct the entered key and try again" occurs because you have an older version of Office Timeline installed but are trying to use a newer key.
To resolve this issue, please download and install the latest version of Office Timeline and then activate using your key.
Close all open instances of PowerPoint.
Download and install the most current version of Office Timeline from: https://www.officetimeline.com/updates.
Once Office Timeline has been re-installed, click on the Office Timeline tab in PowerPoint and click on the Upgrade button.
Complete the activation process using the Activation Wizard.
If Office Timeline continues to show an activation error message, create a support ticket and provide the following information:
The exact error message you are receiving.
Your complete product key.
The version of Office Timeline you have installed. You can find the version number on the Office Timeline tab in PowerPoint by clicking Help > About.