Hi Paul,

When trying to install an App from the AppExchange in Sandbox, you will be prompted to log in to the AppExchange. The login for this must be your login to Production as a System Administrator, otherwise, you will receive an error stating that you do not have the authorization. If you try to use you sandbox login while the URL is set to login.salesforce.com it will state your username or password is incorrect. If you try to update the URL to test.salesforce.com, you will receive an error stating there was a problem with the authentication attempt.

So the proper steps from the Appexchange are as follows:
Select the app
Click "Get this App"
From the Login screen use your Production Administrator Username and Password
You will be asked if you want to install in Production or a Sandbox
From this Login screen use the Sandbox Administrator Username and Password
You will then be in the Sandbox install area and can continue.

Here's a guide that will help you:

• https://login.salesforce.com/help/pdfs/en/salesforce_appexchange_install_guide.pdf

Also look into below information:

Check your profile permission.

Three User Profile Permissions are available for the Force.com AppExchange. These permissions (below), control a users' ability to package, upload, and download listed Apps, or components, from the AppExchange.

These profiles are described below:

1. Create AppExchange Packages

This profile permission allows a user to create packages that can be uploaded to the AppExchange. However, this permission alone, will not allow a user to upload packages.

2. Upload AppExchange Packages

This permission is dependent on the "Create" permission above. A user profile cannot have the "Upload" permission without also having the "Create" permission.

3. Download AppExchange Packages

This permission allows a user to download/install listed apps, or components from the AppExchange.

NOTE: Just like other user profile permissions in the application, the "Create" and "Upload" permissions above require other profile permissions.

"Create" and "Upload" require the permissions below:

- View All Data
- Customize Application
- Manage Users
- Edit HTML Templates
- Manage Letterheads

The "Download / Install" permission requires users to have the following permissions:

- Modify All Data
- Customize Application
- Manage Users
- Edit HTML Templates
- Manage Letterheads

Hope this helps.

Please mark this as solved if it's resolved so that it gets removed from the unanswered queue which results in helping others who are encountering a similar issue.

Thanks,
Nagendra