mbstring is a PHP extension that is required to enable some features inside OptimizePress.
Most hosting/servers include this by default, but there may be some servers where this needs to be enabled.
As this is an advanced PHP modification, we advise you to contact your hosting company to enable this for you.
You can send a request such as that below:
Hello.
I am using a WordPress plugin on my website which requires the mbstring PHP extension to be enabled. Please can you enable this on my server and confirm when this is complete
Thank you
~ Your Name