Can I cruise between Croatia and Barcelona ?

There are a lot of Mediterranean cruise ship itineraries that include Croatian ports and Barcelona. Check any major cruise line and you can see their itineraries.

I would look at MSC cruises first. They are more flexible about embarkation and disembarkation at different points along a set itinerary.

Gwad forgive me, I got curious so I AI-ed it: I got more hits in the opposite direction.

Cruise options from Spain to Croatia
One-way cruises

• Holland America Line (HAL) offers a 24-night Greek Isles, Malta, and Croatia cruise on the ship Oosterdam. It sails from Barcelona to Trieste (a port near the Slovenian-Italian border) and stops in Croatia along the way.
• Norwegian Cruise Line has a 7-night itinerary on the Norwegian Pearl sailing from Barcelona to Ravenna (a port near Venice, Italy) and making stops in Croatia, Malta, and Greece.
• Royal Caribbean features a 9-night cruise on the Explorer of the Seas, departing from Barcelona and ending in Venice, with visits to Croatia and other Mediterranean ports.
• Seabourn Cruises provides an extensive 21-day "Mediterranean Mosaic" on the Seabourn Ovation, which travels from Croatia to Spain, calling at multiple ports. A one-way route from Spain to Croatia might be possible as part of this longer itinerary, so check with the cruise line.
• Viking Cruises runs a "Mediterranean Odyssey" on its ocean ships, sailing between Barcelona and Venice and including Croatian ports.

Another way to do this is by daily ferry. From Dubrovnik there are ferries to Bari, and from Zadar and Split ferries to Ancona.

Then the train (or car) to Civitavecchia via Rome and the nightly Civitavecchia to Barcelona ferry.

At both Barcelona and Civitavecchia the ferry terminals are significantly better located than the relevant cruise terminals. Between Rome and Civitavecchia you would time yourself to use one of the long distance trains, not the commuter trains which the majority of cruise passengers and locals use.