Skype for Business Mac Chat History Storage Location
Open Finder and navigate to ~/Library/Application Support/Skype/.
The “~” sign means your home folder. To find your home folder, open Finder and in the main menu, select Go > Home or press Command (Apple) + Shift + H keys.
It is the folder that has your Skype Name. 

An app restart may be required after making modifications. 

Source: https://support.skype.com/en/faq/FA10888/how-do-i-manage-my-conversation-history-in-skype-for-mac

Also, try this location: /Library/Application Support/, as there is a difference when the tilde sign is used.

Furthermore, the stores file can be found in ~/Library/Containers/com.microsoft.SkypeForBusiness, which is the user folder according to This Post