A lot of people facing issues to install an app on the sandbox.
Steps to follow:
- First login to Production Org with your account (ie. not sandbox). — Check this first — You account is enabled with the “Download AppExchange packages” permission to install AppExchange apps.
- Browse the AppExchange site to find the application you want to install.
- Click Get It Now from the application information page. — If the listing does not have a Get It Now option, it may be a partner app that you can download and use outside of Salesforce. Contact the publisher for more information.
- You will be asked if you want to install the app in Production or Sandbox
- Choose sandbox and you will be automatically redirected to the test.salesforce.com
- Enter in your salesforce sandbox credentials
- Now just follow on-screen instructions.