The "Multibyte string PHP extension" is a requirement of WordPress, the system used by WooCommerce. Learn more about PHP extensions.
In most cases, WordPress should preclude you from configuring additional plugins unless this has been configured. However, in some cases a WooCommerce account will function and spontaneously report a related error.
The error will appear in your log messages as:
Fatal error: Uncaught exception 'Exception' with message 'ShippingEasy needs the Multibyte String PHP extension.'
Learn more about viewing your error log messages.
To resolve the error, you will need to contact your web site hosting company. This forum thread from WordPress Support has more information.