If Correos does not accept the package, it may be because the measurements do not meet the carrier's conditions.
The maximum weight allowed is 30 kg per shipment including packaging. As for size, once packed, the sum of the length, height and width cannot exceed 210cm. The longest side of the packaging cannot exceed 120cm. The seller must select the correct weight in his advertisement, taking into account the packaging.
In case the weight is incorrect, the seller must cancel the transaction with the button found in the Sales > In progress section. The refund of the transaction amount will be made automatically to the payment method used by the buyer. The amount will be reflected within 3-10 business days depending on the payment method used. For more information about refunds, please check the following article: When do I receive a refund.
Once done, the seller will be able to edit the product and modify the information regarding the weight, selecting the correct range (between 0 and 30 kg). To edit the weight of the product, the seller must go to the ad and click on the pencil icon in the upper right corner. One of the editable sections corresponds to "Shipping" where you can choose the range that fits the product and once selected, click on the "Update product" button.
Once the seller has edited the weight in the ad, she will have to notify the buyer to send a new shipping request so that a new Correos code can be generated.
For more information you can consult the following articles: Size and Weight and Cancel a shipment once the purchase is confirmed.