You can't tax it online:
Change of tax class
If you�re changing the tax class from private to disabled, provided you have the whole Registration Certificate, you can tax at a Post Office� branch.
If you only have the New Keeper Supplement and for other tax class changes eg from disabled to private, you�ll need to tax at your nearest DVLA local office. Write the new tax class in section seven of your Registration Certificate. If using a New Keeper Supplement, you�ll also need to fill in a V62 �Application for a Vehicle Registration Certificate (V5C)�.