Make sure the microflow which you did mapped/configured exist in your application or not, if it’s not exist then only it will throw a microflow could not be found error.
May be you might have created and deleted, re create the same and map it should work.
I have created it several times and configured it correctly many times. Then it doesn't work again in the browser
Is the microflow visible for Anonymous user? See the properties pane of the microflow.