There is an unverified transaction stuck at the till.

Swipe from the left edge towards the centre. Select Event history and press Check. If the cashier recognizes the status of the transaction, the cashier will report it after checking and clear the list.

It is possible that the cashier cannot find the transaction and is therefore stuck and refuses to exit with a check. In this case, you need to go to your Solmio settings and find and select the option Clear the cache and then OK.