Not all features are available in the Professional Edition license of Salesforce®. Professional Edition users do not have access to creating the button and adding it to a Page Layout. However, you can build the button syntax and then copy and paste it into the button created on the Master Object. Thus, you will be creating a solution with the Solution Manager and then manually creating a Conga button. For more information about this process, see
Create a Conga Composer Solution Manually. This edition does not support the Solution Manager.