There are two common errors that you may experience with the Winehub POS Connect extension and both of them are realtively simple to rectify;

POS Connect requires the most recent version of Shopify POS to be installed to function correctly. If you are experiencing any issues, please ensure you update to the most recent version of Shopify POS before proceeding with any other solutions
Winehub POS Connect - App Failed to Load
This error generally presents when there is an internet connectivity issue with your POS. Check the 'connectivity' icon on the bottom left of the Shopify POS menu (see screenshot below). POS Connect will become available again if you swap internet connections or when normal internet connectivity is resumed.
Authentication Error - Store Connection Unavailable
You may receive an authentication error if the POS User does not have the correct permissions in Shopify settings. You can rectify this using the following steps;
1. Go to Shopify > Settings > Users & Permissions
2. Open the POS user and scroll down to the Point of Sale Section
3. Click on Show Permissions and check that Use apps that work with Shopify POS is 'Allowed'
4. If not, use Manage POS Roles to adjust the permissions of this role to allow Use apps the work with Shopify POS