You can learn on a new, or second-hand, keyboard so long as it (ideally) has these 3 main features.
3 RECOMMENDED PIANO KEYBOARD FEATURES
1. The keys are standard size (i.e. not children's size)
2. The keyboard has at least 61 keys (counting both black and white keys)
3. It is touch sensitive (i.e. it plays louder when keys are pressed harder)
FOR BEGINNERS
We have a few students who are using keyboards that have fewer than 61 keys and/or a keyboard which is not touch sensitive and, while this may limit your playing experience, you can definitely start making progress on the course with this type of keyboard, and then upgrade to the recommended type of keyboard whenever you get the chance.
My recommendation for a good, low-priced beginner keyboard is the Yamaha NP15
Find out why, here: https://decplay.com/best-piano-keyboard-for-beginners/
ePIANOS.CO.UK
For more advice about choosing your piano set up, including the best piano keyboards, seats, stands and pedals, etc, I recommend students in the UK to contact Chris or David at www.epianos.co.uk/decplay.
They have many videos comparing different keyboards, and have a good range of new and used keyboards, with UK delivery.
They also have lots of videos giving helpful tips.