Missing style.css Stylesheet

You will usually encounter this error if you uploaded a plugin or a content template to the WordPress theme directory (WordPress Dashboard >> Appearance >> Themes).

Please ensure that when installing a plugin, please go to Plugins >> Add New 

https://optimizepress.zendesk.com/hc/en-us/articles/201328503-Install-OptimizePress-Plugin-via-Wordpress

If you are going to install a content template however, please go to OptimizePress >> Create New Page >> Upload a content template in zip format

https://optimizepress.zendesk.com/hc/en-us/articles/203704616-How-to-install-a-content-template