If you are wondering how to define minimal order purchase value for selected groups of customers just take a look on the video below. When customer will not meet requirements (limitations) that you will define on module configuration page - then  PrestaShop will display information about that and of course block possibility to order stuff. Video below shows configuration process of the module, how it works, how it displays information message etc.

Text instructions

• 1) log in to your shop back office
• 2) go to modules > modules section
• 3) search for "minimal order purchase"
• 4) install module
• 5) open module configuration page
• 6) define minimum order purchase value in input fields near each customer group
• 7) save changes