Why am I getting the 'Partner record exists for this client' message?

If you receive the following message when trying to create a partner record this means that the client record you are trying to add the partner record to was at one time related to a record which has been deleted.

To correct this, navigate to the client record, the Personal link then the Notes tab:
Click into the System Partner Reference field and you will be prompted with the following message.  Click on Yes and you will now be able to Create partner record: