You´ll have to select the subject of your complaint.
Please note that there are specific windows for all the different kinds of shipments.
On the Correos webpage (link above) select the bottom link for international shipments and the one above that for national ones.
You need to look up the guarantees of the specific postal service you used and the specific window to appeal if you are looking for reimbursement of shipping costs and/or compensation for value of content of the package.
Here´s a LINK to the international tariffs and the cover/guarantee when a package get lost or sent to wrong address or just very long underway (demora). Choose the link at the bottom of the page.
DT stands for the return of the shipping costs you have paid and DEG is a monetary unit to express the compensation due.