Presume you are getting 'Diesel addative minimum level' alarm which is for the Eolys fluid.

The tank for this is located next to the fuel tank underneath the car & can be filled fairly easily. 5 liters is about £125 from Peugeot/Citroen dealer.

But after filling you will still get the alarm message unless you get the emissions ECU reset on the car.

Also if you are low on fluid it will also be time to change/clean the Particle Filter in the exhaust.

If you are willing to get your hands dirty & get under the car it is a DIY job.

Do a search on this site for Eolys, there are instructions in various threads on what to do.

A garage will charge upwards of £500 to do the complete job including fluid & filter.