From READING THE MANUAL:
3.6 Refilling with water
Note :
When the water in the boiler has been used up, the red low-water warning lamp (A6) will illuminate. The safety cap (A2) cannot be opened while there is any pressure at all in the boiler.
Manual Here:
https://www.karcheroutlet.co.uk/Manuals/Steam%20Cleaner/K%201105%205960-099.pdf
RTFM
November 2015