When installing App to SharePoint sites, if you company has enable Approval (Microsoft article on App Approval) then you will have to complete the App Approval before you can add it to a Site. The instructions below will take you through the process of adding the app.

Prerequisites:

1. You must have full control access to the site

2. The user account used to install Read and Understood must have a valid email address and mailbox.

Installation

Note: If you have linked to this page because when trying to add the App you see 'Request It', then skip steps 1-3 below.

Click the Image below to open Microsoft AppSource and locate the App
From the Microsoft AppSource page, click 'Get it now'
From the SharePoint Store Page, Click "Switch to the Classic App Store"
Click the ‘Request it’ button:
On the reason for request screen, add a message and click ‘Request’
Result: A request will have been made to a Tenant SharePoint Admin to Approve your request. If you have SharePoint Admin access, you can continue with these instructions to approve your own request.
In the browser, navigate to the SharePoint Admin Center.
From the SharePoint Admin center page, click 'More features'
From the More features page, local Apps and click 'Open'
On the next page click 'App Catalog'
From the App Site, click 'App Requests' in the site navigation. Should then see the Read and Understood App Request.
Edit the App Request for Read and Understood.
Update the App Request and set it to approved. Before you click Save, click the link below 'Click here to view the App Details'.
Result: A new tab will open in the browser. Now click Save to complete the Approval.
Next, you must add the Read and Understood App to the App Catalog Site (keep the page open). In the browser, click the tab that opened in the previous step.
Click the ‘Add it’ button:

Important: if the 'Request it' message is still displayed. Go back to the Apps Page from the previous step, click 'More Features' > Configure Store Settings. If App Purchases is set to No, then set it to Yes to add the App.
Follow on-screen instructions to complete the process of adding the App to your SharePoint Tenant
You can now add the App to a SharePoint or Microsoft Teams Site. Navigate to the site you wish add the App.
From the top Right corner of the site, click 'Settings' > ‘Add an app’:
On the My Apps page click "Add" the Read and Understood
On the trust screen, click ‘Trust it’
Result: After a few seconds the app will appear in the Site Contents pages as shown below: