Basically, they (the UK in this case) can use the draft schedules they submitted to the WTO... but they might have to offer compensation (to some countries) later after the [modified] schedules are ratified. Which in the case of the UK looks unlikely to happen by March 2019:
An article back in October (talking about the schedule for goods) noted that
The UK’s Department for International Trade has said it is not uncommon for members to object, but a sense of urgency exists if the UK needs to rely on WTO rules for trade with the EU if there a hard Brexit.
The UK will now be forced to wait for 90-days before the negotiations can begin, according to article 28 of the General Agreement on Tariffs and Trade. That means that the UK will not be ready by March 2019.
Trade will not come to a halt during the waiting period, but any new scheduled deliveries will be uncertified. A similar scenario played out during successive waves of EU enlargements, most recently with Croatia in 2013. The issue is often resolved by offering compensation to WTO members that make the case that they are losing market access from the revised schedules.
And another article from January this year noted that Taiwan has already raised objections to the new (December) UK schedule for services in the areas of air and financial services, e.g.
there were several areas where the EU schedule included a requirement for establishment in the EU, Taiwan said.
The United Kingdom changed ‘EU’ to ‘UK’ for these entries in its draft schedule. This would appear to reduce market access, as it reduces the geographical scope of establishment.”
If Taiwan and potentially other WTO members press Britain for improvements to the text, Fox may need to offer them compensation by opening up trade in other areas, although any such liberalisation would apply across the WTO.
Britain is facing similar objections to its goods schedule, with widespread dissatisfaction among agricultural suppliers.
Failing to reach rapid agreement may be a bureaucratic headache and add to criticism of the Brexit process, but it will not affect the Brexit timetable; many WTO members trade without finalised schedules, on the understanding that they are trying in good faith to reach an agreement in the meantime.