How can I subdivide this if I cannot use the loop tool cut? So I want to subdivide, then use LoopTools and finally circularize that section to include that piece of the model in the object (the one in blue on he photo and previously modeled), but I can't. Please, any suggestions?
Thanks in advance to everyone who took the time for reading this!
At least one of your polygon has more that 4 vertices ; this one has 5 vertices:

You can't use the Loop tool because you've got NGons (more than 4 vertices). You can use the Knife Tool instead.