1) Don’t complete If the developer is at fault. You force them to mend all the flaws before completion or else practice a retention at completion until the outstanding flaws are fixed (after completion that is). Your lawyer will have to send burofaxes or requerimientos notariales to justify and demonstrate they are not at fault by refusing to complete because there is one or more legally justified reasons not to do so.
2) If the developer is not breaching an essential clause of the contract but you without any justified reason don’t want to complete (because you hoped to sell on the property before completion for example) you risk losing 10-30% of the funds already paid (the amount varies and depends of the contractual clauses). You cannot possibly refuse to complete without a legally justified cause.
What is it that you don’t understand ? It’s very straightforward.