A WTO Goods schedule lists maximum tariffs a country agrees it can charge.
A schedule is 'certified' by the @wto Director General only if there are no objections.
Note: A country **CAN** operate with an uncertified schedule.
They may. By doing so however, they would be accepting everything in the schedule as a fair reflection of the UK's obligations post-Brexit.
More likely they'll at the least put certification on hold for a while.
Not the end of the world. Two things.
1) An uncertified schedule makes it more likely someone is at least open to a WTO dispute against the UK where its new commitments fall short of the EU's overall commitments.
2) An uncertified schedule also makes negotiating bilateral or plurilateral free trade agreements a little bit more complicated because there is ambiguity on where to start.
A WTO Members schedule is often the 'baseline' against which negotiating outcomes are compared.