Calculating exposure time to inactivate coronavirus by UV light
yes, there are several errors in your calculation.
Germicidal lamps specify an effective power output in UVC. This lamp
https://www.osram.de/ecat/PURITEC%20HNS%20UV-C-UV-C%20lamps%20for%20purification-Ultraviolettstrahler-Industrie-Spezialbeleuchtung/de/de/GPS01_1028570/ZMP_1022255/
consumes 16W of electricity but has a UVC output of 4W. You need to calculate with the 4W value.
Then you need to calculate the intensity of your irradiation on the surface you want to inactivate the virus on. The distance from the lamp to the surface is key, as the 4W UVC power remains contant over the surface of the light cone that shines from it. In the lamp's data sheet it says that this is valid for up to 3m from the lamp. (after 3m it most likely decreases due to other effects.) So you need to calcuate the distance from your lamp, and you need to calculate your powerdistribution over the surface of the sphere/cylinder (depending on your geometry), with the lamp in the center. It does not do to calculate the surface of the room as a whole. Given the irradiation intensity on the surface of that sphere you can now calculate the time needed until the UVC output of the germicidal lamp manages to reach the 90% inactivation dose for a given virus. For corona viruses different values for that dose are given in different papers. Some say 10.3mJ/cm^2 for 99%, some quote 0.6mJ/cm^2 for 90% inactivation. This one gives a good overview: https://www.nature.com/articles/s41598-020-67211-2.pdf
I recommend to choose a simple geometry for calclation of the irradiation power. Living rooms are quite tricky. Spheres are better.