During the Private Sales period, only users/wallet addresses that won OP3N - PikoZoo campaigns on Twitter, other social media channels... and were added to the Whitelist list can buy NFT(s).
Please double-check your wallet address to make sure it's in this Whitelist list first. In case you're in but there is no option to buy NFT, please contact OP3N Support Team with the details of wallet address and screenshot of problem if any.