Some Fields in the system, such as Account.Source and Account.Owner, can’t be changed in the Custom Fields page.
This is because they are used by other parts of the CRM system, so changing them would cause malfunctions. For example, the Source is copied from the Company Account when you create an Opportunity, and the Owner defines security permissions in User Permission Levels.
To create new custom fields, please read this article: How to create and manage Custom Fields?