If compilation mode in your Magento is enabled in your Magento store, you have to turn it off, otherwise your store and admin site would be inaccessible after installing the extension.

To disable Magento Compilation, go to System–>Tools–>Compilation.

Also, make sure you have backed up your Magento files and databases in case things go rogue.

It is highly recommended that your configuration cache is turned on to prevent early installation, as it could be a reason for errors.

Go to System –> Cache Management 

Switch Action to Enable and hit Submit.

After successful purchase, we will provide you with the extension's zip file. Unzip the extension into a directory of your choice

We recommend using FileZilla for uploading the files. Enter your store's Host IP, User name and Password, then hit Quickconnect 

To quickly install the extension, navigate both your local and remote site to app, right click on it in your local site and hit Upload

You should refresh Magento's cache to provide Magento with the ability to register a new extension.

To prevent an “Access Denied” error, you have to re-enter to the backend. Log in with a full administrator account to get the new permissions.

Now you should check whether your website and your new module works as expected.