What to wear Spain, Portugal.

The only "must have" I can suggest, is shoes with grippy soles. The streets and walkways in Lisbon and Porto will be hilly, and can be very slippery.

Check the weather before you leave and adjust accordingly.

In 2019 we were in Portugal for last two weeks of October. The heaviest layer we wore was a fleece or sweatshirt.

You will need clothes in layers, since it will be chilly at times and warm at times. Plan on wearing long sleeve shirts, trousers (I like kakis) but have a sweater and coat to wear when it is chilly.

We were in both last September and never needed a jacket, but we only went as far north as Baiona and as far south as Óbidos We didn’t pack any different for that trip than any other trip. If heading further north in Spain a light jacket might be handy.

I am able to pack what I need without a luggage weight issue. My wife, not so much.

In the immortal words of David Byrne, "Same as it ever was."

I suspect the issue here isn't so much not knowing what the climate requires.

The cobblestones are hard on your feet in Portugal especially. A thicker sole, like you get with a hiking boot or shoe is good for those. If it has been raining, those stones get really slippery.
I would just wear what ever you wear at home, but look at the weather forecast for those regions. It may be chillier in Spain by Oct, especially mornings and evenings, so a jacket is good to have.

I wear what I wear every day—cargo shorts and a polo shirt.
I refuse to let the weather dictate my clothing choices.