You might need to sell an item on this Private account first then:
Go to "My eBay" then "Account," then "Personal information". Select "Edit" next to "Account type" and choose "Business account". You may need to sign in again.
Provide your business information, including name, contact details, and returns policy, then select "Save.
You may need a laptop / PC and use Chrome for the above, and not the app or mobile browser, which can be restricted in their options and features.
@ars4496