I have problems with some unwanted objects that appear only at the time of export (at least exporting to powerpoint), and I have already tried to purge them and nothing. I’m very new to this.
And when i try to see whats going on in sketch up, the object is gone, but it stills appears when exporting.
You have loose geometry that has not been grouped or made into components. It is best to only have groups and components. It is also best not to tag loose geometry. Only tag groups and components. You have a lot of components from the 3D Warehouse. You should be careful about choosing these. It is recommended to open them up first in another file to inspect them and correct them. Hopefully, @DaveR can look at this with his tools and give you some detailed adivce about all of this and more.
What version of SketchUp are you using? Your profile indicates you are implies SketchUp 2024 desktop but your model looks like it came from SketchUp Free (web).
When you select a menu Download>STL you will get a your model exported to STL.
It is just a different naming, and different location in a menu.
You will get a same result if the STL sub-menu would have been placed inside the Export menu. Actually I have no idea why the Sketchup developers chosen to make users confused by placing it to other menu…