It is possible to buy a domain name that is already taken, but it will require some additional steps.
If the domain name you want is already registered to someone else, you can try to contact the current owner and see if they are willing to sell it to you.
If the owner is not interested in selling the domain, you can try to find a similar domain name that is available for registration. You can perform your domain search here: https://encode.host/cart.php?a=add&domain=register
Alternatively, you can wait until the current registration for the domain name expires and then try to register it yourself. Keep in mind that domain names are typically registered on a first-come, first-served basis, so someone else may register the domain before you if it becomes available.