Ensure that you have web check-in enabled in the system. You can enable this by doing the following:
First tap on the Hamburger icon and once the drawer menu opens, tap on Queue settings. Check if you have opted for "Inherit Business Settings" or "Customize this queue".
If you have opted for customizing this queue, remain on this screen and select the checkbox "Allow web check-in". Also ensure that the check box "Allow business check-in only" is not selected.
If you have inherited business settings, tap on Business settings from the drawer menu and select the check box "Allow web check-in". Also ensure that the check box "Allow business check-in only" is not selected.
Lastly tap on the hamburger icon, and once the drawer menu opens, tap on Queue. Tap on settings on the top right corner and check if your Queue is turned On. If not please turn on the Queue so that your customers can check-in.