It should be, but if I was you, stick to a big company like AVIS or HERTZ.
Alamo car hire (part of nationwide vehicle hire) in Spain totally dumped on me we had pre paid the entire rental AND deposit, but thier pencil necked airport rep refused to give us a car without seeing a credit card. This was at 1am in Allicante. We had to get family out of bed to drive nearly 200 miles round trip to rescue us, and to this day I have recieved no apology, and no refund (£300 plus, lost)
Tread carefully, and really make certain you are getting your car under your terms. If possible arrange pickup at a time when an english speaking rep can be found, or at least phoned should things go tit’s up.
We hired from Avis next trip, 100% excellent service, and got bumped up to a medium size a Seat Cordoba in as good as new condition because the small car we had paid for was un-valeted when we arrived.

It’s pretty much worldwide that you need a credit card to hire a car. The reason is for security and insurance. I’ve never come across a company that will let you hire a car without a credit card, and I’ve travelled quite a bit in my life.
A credit card is handy to have whilst travelling in Spain. It saves taking too much cash with you. You also get a good rate of exchange if you need to buy goods. Once you get home you can cut it up or shred the card.
The well known car hire companies such as Avis and Hertz are great people to hire with but expensive, compared to hiring in Spain with someone like Carjet Iberia. You will save as much as a third of the cost.