Subdomains do not exist by default, you need to initialise its Solana account in order to create it.
Go to https://naming.bonfida.org/#/domains-registration and search for your sub-domain. For example, if you own bonfida.sol and want to create help.bonfida.sol, enter help.bonfida in the search bar
After pressing Enter you will see your subdomain even if it has not been registered yet
To initialise your subdomain, click on Edit and enter the data you want to save in the account
Click Edit and approve the transactions. The first transaction will initialise the subdomain, and the others will edit its data with your input.