I tried changing the SMBIOs to iMac18,3 (Retina 5K, Late 2017) which made the system boot still but yet the problem is still there. I have never started an installation of macOS and I'm getting this error like the macOS installation has started already. Could that give a clue?
P.S: I can send the log file if needed.
Update: After trying to save the log in my PC, I've noticed that the folders Applications, Desktop, Documents, Downloads and a "myDocuments" folder have appeared in the Favorites section, it's actually like the macOS installation has started, but I've never even started it!