Payments To The Tax Office - Possible to Pay in Instalments?

In most cases yes, but the way to apply for fractioned payment differs.
If still within the ´voluntary payment´ window, like with RENTA Income Tax before the 30th of June, you can tick the relevant box for fractioned payment. Outside the ´voluntary payment´ window, for example if the Tax Office notifies you that you have to pay an additional amount of income tax over a return that you´ve filed sometimes years ago

or

If you´ve received a summons to pay any kind of invoice, originally issued by any of the government bodies, like the Catastro or DGT/Trafico (fines) and now referred to the Agencia Tributaria because of non-payment, you will have to apply for payment in installments.
You can do so online, or in person. Would have to provide documents to support your claim that you are not able to pay all at once.

If the matter has gone even further and the Tax Agency has placed an embargo, you cannot request payment in installments but in many cases the demand for payment will come with a payment schedule.
In our experience, although the regulations state it´s not possible to pay in installments when assets have been embargoed, it´s still worth a personal visit to the tax office in your area, to try and negotiate a payment schedule that is more convenient.

Info from the Tax Agency, in English

HERE

Please note: The information provided is based upon our understanding of current legislation. It is not legal advice but is provided freely to enable you to be properly informed. We recommend that if you are considering taking action, you should seek professional advice.