When you purchase a commercial plugin, such as Solid Backups, Solid Security Pro, etc., you cannot install it directly from the WordPress Repository.
Instead, you'll need to upload the plugins to your site.
Note: Do not unzip the download file. You will upload the zip file in your dashboard.
There are two ways we'll cover here that you can use to install downloaded plugins onto your site:
Installing Downloaded Plugins Through Your Dashboard
To install a plugin on your site that you've downloaded from somewhere, you'll need to go to Plugins > Add New.
By default, this will take you to a page where you can search the WordPress Repository for plugins. However, at the top of the page, there is an Upload Plugin button.
Once you click the Upload Plugin button, an upload option will appear where you can choose your file.
When you click the Choose File button, be sure to select the zip file of the plugin you are installing. You cannot upload the plugin if you have already unzipped the file.
Once you've selected your file, click the Install Now button.
You will then be given the choice to activate the plugin or return to the plugin installer.
Installing Downloaded Plugins Through (s)FTP
To install a plugin to your site using an FTP client such as Filezilla, you'll first need to unzip your download file.
You'll then log into your FTP client using your FTP credentials. You can get these from your host if you are not sure what your FTP credentials are.
Once you've logged into the site through FTP, you'll need to go into your site and to /wp-content/plugins/. There, you can upload the folder created when you unzipped your download file.
When the upload has completed, log into your site and go to your Plugins page. From there, you'll be able to activate the plugin you just installed.