The same message would be displayed with a good but discharged 12 V battery, so before replacing it, try charging it with a battery charger.
Because we'd been driving our i3 only ~150 km per month during the pandemic, its 12 V battery wasn't being charged enough to offset the 12 V vampire loads (e.g., burglar alarm, keyless entry system, telematics module). This message was displayed in January. I charged the original 12 V battery, now 7 years old, and haven't seen that message since.
Of course, your 12 V battery might be bad, so it makes sense to buy a replacement to be ready when needed. I bought a replacement over 2 years ago when older i3 12 V batteries began failing but haven't needed it yet. I charge it 2 or 3 times per year to prevent it from self-discharging too much.
I can't advise you about which battery to buy in Spain because the batteries available in the U.S. are not available in Spain.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE