3B Forms require that your Salesforce instance has at least 1 active Site. The app uses Salesforce Sites to display forms to the web (for users who do not have access to your Salesforce org). If you do not have a Site license, please contact your Salesforce representative or submit a case at CASE URL.
Creating a Salesforce Site
The process is the same for both Salesforce Lightning and Classic interface.
- Navigate to Setup (top right hand corner)
- In the quick find box to the left, type “Sites”
- Select “Sites” option under “Develop” (“Sites and Domains” for Lightning)
- Type the desired unique domain name. Click on “Check Availability”
- Read the “Sites Terms of Use” and check the checkbox to indicate you have agreed
- Click on “Register My Salesforce Site Domain”
7. Once you have registered a domain, you need to create a new “Site”. Click on the “New” button 
8. Complete the form
- Give a name to the site (Preferably “Forms”)
- Add Site Contact – this is the contact that will receive notifications about errors/issues if any
- Select “InMaintenance” in the “Active Site Home Page” field, unless you have built a Visualforce Page that you want displayed on the Site permanently.
- Select “Active” to be checked
Thats it! You have just set up a new site. Depending on how many licenses you have purchased from Salesforce, you may be able to create multiple Sites. You can also refer here for further instructions as provided directly by Salesforce.