Professional Accounts are sold under the business account banner.
Despite this, for some arbitrary reason Professional Accounts have been internally categorised as personal not business accounts.
As a result Professional Accounts cannot be linked to a Personal Account.
As a sole trader I have no interest in purchasing the minimum 3 Business user seats required but still require the functionality to keep personal and professional files separate (whilst syncing both accounts to the same device etc).
Please enable account linking for Professional accounts.