The most likely reason the warning is coming up is because its not detecting the tank.
With the machine powered off, remove and refit the tank, when inserting the tank it should be pushed right back until you hear a click (at which point it locks into place). Once the tank is correctly inserted power the machine on, if the same error shows, this could suggest a sensor fault.
At this point follow the maintenance schedule on page 17-19 of the manual (a copy can be found here), if after maintenance the unit still doesnt detect the tank, I would look to arrange for its repair, a list of repair centres can be found at the bottom of the page found here.
Please let me know if you require anything clarified.
This is not something I would suggest you do yourself, a home repair of this nature can lead to electrical fires as this is a high energy appliance.
I will opt out and let another engineer respond who may be willing to talk you through the steps required to strip down the unit, apologies I should have set expectation at the outset.
Like the technicians at the service centres referenced on the Delgoni website, I support and service a range of manufactures, Delonghi is just one of those.