Error at check out shipping not available and payment couldn't be processed

We have a discount code and shipping free over £50. people ordering total is £55 when discount applied it is £44. it wont let them pay or check out. 2 error messages “shipping not available” items in cart do not meet price and weight requirements” and then payment could not be processed items in cart have changed. we can’t see how to fix this any ideas would be very welcome urgently

Please ensure that you have configured the shipping rate for orders amounting to £0-£50