Barcelona-Guangzhou-Barcelona

If you travel like Barcelona-Dubai-Guangzhou-Dubai-Barcelona, you won’t be eligible for the 72 hour free transit in Guangzhou.
But if you travel like Barcelona-Dubai-Guangzhou-HK-Barcelona, and stay in Guangzhou for not exceeding 144 hours, you will be able to use the 144 hour free transit, and then no need to apply for a visa.

Thanks for your answer. Then the problem could come from airline checkin as they may not be aware at origin of all the different cases, so they may deny the travel to be in the safe side. I could also do Barcelona-Dubai-HK-Guangzhou and then Guangzhou-Dubai-Barcelona. in IATA website I have read this:
"Passengers with a normal passports arriving at Chongqing (CKG), Guangzhou (CAN), Guilin (KWL), Haikou (HAK), Kunming (KMG), Qingdao (TAO), Sanya (SYX), Shenzhen (SZX), Weihai (WEH), Xi An (XIY) and Yantai (YNT) can obtain a VOA if they have confirmation that a visa has been approved before departure and:
- that a visa will be issued on arrival; and
- passport contains at least 1 blank page for the sticker-type visa; and
- a sponsor (approved by Chinese authorities) meets passenger on arrival.
Note: A representative of the arriving carrier must be informed before arrival of the flight."

Do you know how can I get "confirmation that a visa has been approved before departure"? How can I get the VISA approved if I do not have a VISA...strange.
Thanks again

Yes, BCN-DXB-CAN-DXB-BCN requires a visa, but BCN-DXB-CAN-"anywhere except China or UAE" -BCN you may transit without visa for 72hrs.

For transit without visa, I believe Guangzhou is currently limited to 72 hrs (starting at 00:01 on the day following arrival), although there are rumours that 144 hrs may be officially announced soon. Also I believe you must arrive and leave by air at Guangzhou. Train or boat is not permitted. Again, that may change at some point in the future, but I would be hesitant to rely on that at the moment.

You cannot normally get a visa on arrival. So don't refer to "visa on arrival" when you are checking in at Barcelona or boarding at Dubai, always say "transit without visa" (assuming your itinerary is compliant).