There is a Facebook bug making it so you can't remove Facebook Page tabs, last verified December 17th 2020.
Facebook says it is working on a solution to be able to remove tabs. Facebook has been aware of this bug and has said it is "assigned" since September 2020.
To be notified when Facebook fixes this issue, on the bug report webpage you can click the blue Subscribe button in the upper right. Subscribing to updates on this bug may increase the chance it is fixed sooner:


Directions if Facebook bug has been fixed:

To remove a tab, on your page click on Settings (upper right of page) > then click on Templates and Tabs (upper left of page) > On the lower right of this page you'll see a list of all page apps.

On the tab you want to remove, click Settings > Click the button labeled On, this will turn the tab off, and removes the tab from your page.