The CIA might very well have had information that Las Ramblas could be a target.
Maybe the Spaniards increased security. Maybe not. If the CIA had provided a date, along with an assessment of the source of the information, then this could have made a big difference.
Back in the 1980s and earlier, airplane hijackings were popular among terrorists. Every terrorist dreamed of hijacking an airplane. Some actually pulled it off. And intelligence agencies at the time received reports such as, "we are going to hijack an American aircraft when we are able." Not quite enough to go to DefCon 5. True, using a car as the weapon is a lot easier than hijacking an airplane, but unless you have a name, a date, or something else to go on, it's not particularly helpful.
Some might bring up the August 2001 intelligence that al Qaeda was planning something with airplanes in the near future. That's somewhere in between "we will hijack an airplane when we are able" and "Abdul is going to drive a car down Las Ramblas on August 16." Some actions could have been taken in 2001 but weren't. And the rest is history.
The point here is that the implication, I think, is that we had info, shared it, and the Spaniards did nothing. This is not necessarily an accurate reading.