How can I fix?

I'd advise you to do a simple Google search for - say - "Citroen C3 battery light flashing"

You'd have found things like this in the first few results:

https://citroenc3owners.com/petrol-citroen-c3/c3-won-t-start-but-turning-over-battery-light-flashing-t348.html

https://citroenc3owners.com/citroen-c3-tips/c3-battery-light-flashing-t132.html

acp
May 2017