Can I connect a keyboard to my iPad 6th generation 2018?

The majority of USB keyboards can be used with iPad when using a USB Adapter - although some extended character-set and special function keys will not be relevant/supported.

As your iPad6 has a Lightning port, to connect a wired USB keyboard or other USB devices, one of two Apple adapters are recommend:

• Lightning to USB 3 Camera Adapter

https://store.apple.com/uk/xc/product/MK0W2ZM/A

Don’t be fooled by the name; this Adapter is by far the most flexible for connection of supported USB devices to your iPad - and allows simultaneous connection of your Power Adapter.

• Lightning to USB Camera Adapter

https://store.apple.com/uk/xc/product/MD821ZM/A

This adapter doesn’t support simultaneous connection of power - and only supports USB 2.0 transfer speeds.

If considering a Bluetooth keyboard, you would be best advised to choose a model that explicitly lists compatibility with Apple devices (a.k.a., MFI).

you can connect all bluetooth keyboards, if you mean keyboards which require an usb dongle then you can only use keyboards which specifically state ipad compatibility