There are T & C's items have to conform to, to use the Click & Collect system, maybe your item falls short?
The seller has to set this as an option when listing, although they're confirming they added this, maybe they genuinely forgot?
Now the item has been purchased the seller cannot add C & C, the only long way round this would be to cancel the order, which refunds you. Relisting the item selecting Click and Collect, ensuring it complies to the T & C's, for you to buy again.
@shee6896_xceug