It is not possible to cancel or withdraw your bid. Each bid placed on Catawiki is binding and will remain so unless it is outbid. If the lot concerned was fixed with a reserve price, that lot will not be auctioned off unless its reserve price has been met.
You can find more information about reserve price or minimum prices here.
If you end up being the highest bidder and you win the lot, you are required to pay for it. You are able to make this payment on your 'My won lots' page under the 'Unpaid' tab. If you do not make this payment, you will be blocked from placing any further bids on Catawiki.