Enable the Downloadable Products Proxy in Community Hub

Add field a field to the Entity Page Layout:
1. From Setup, click Create | Objects.
2. In the Custom Objects list, click Entity.
3. In the Page Layouts list, click Edit next to Entity Layout.
4. If it does not already exist, we recommend you create a new Community Hub section under the Accounting section.
5. If they are on the page layout, move Payment URL (Order), Payment URL (Cart), Checkout Page to the Community Hub section.
6. In the Community Hub section, add Download Proxy Page.
7. Click Save.
  
  Repeat these steps for any custom Entity Page Layouts.
  You may need to give profiles permission for these fields.
Add the download proxy URL to the entity:
1. In the tab bar, click
  App Launcher, or in Salesforce Classic, click
  Nimble AMS, Staff View, Button, All Tabs.
2. Click Entities.
3. Select a list view—like All Active Entities—and click Go!
4. Click Edit next to the entity of the Community Hub for which you are enabling the proxy.
5. In Download Proxy Page, enter the relative path of the page that validates user access to downloadable content. For example: nc__download.
6. Click Save.
7. Repeat these steps for all entities.