Do you have to remove the iPad case to use the keyboard

You can use a Bluetooth keyboard with iPad whilst a case is fitted.

However, if you refer to the Magic Keyboard for iPad, or Smart Folio keyboard, you won’t be able to use the keyboard while the iPad is in a case - unless the case is specifically designed for use with these keyboards. These keyboards connect to the iPad using the Smart Connector contacts (of which there are three) on the back of the iPad chassis - and the keyboards attach to the iPad with an array of magnets.

Most cases, unless explicitly designed, will obstruct the Smart Connector contacts and interfere with the magnets.

Well, which case do you have? Just a normal case or a Magic Keyboard/Smart Folio. If you want to use the on-screen keyboard with a Magic Keyboard/Smart Folio case then you have to take the case off. But if you're using a normal no keyboard case you don't have to remove the case.

If you are using a Bluetooth keyboard you can have the case on while the keyboard is connected.