Installing an extension is a three-step process that should take place during off-peak hours. Before the extension is installed, your store is put into maintenance mode, checked for readiness, and backed up. After the extension is installed, it must be configured for your store according to the developer’s instructions.
https://docs.magento.com/marketplace/user_guide/buyers/install-extension.html
Common Installation Issues:
- The customer using the incorrect access keys to the repo.
- The customer purchasing the incorrect package (EE instead of CE or vice versa)
- The customer not purchasing under the correct account.
- Incorrect default repo selected in the customer's Magento.
- Web Setup Wizard (component manager) not configured correctly.