First rule: for deleting any object, you need to select its category. You cannot delete plants if you are in the objects tab.
Second rule, select the element and hit on the little circle that identifies it. Or, hit on the bin on the top right.
Download Lumion software here: https://support.lumion.com/hc/en-us/a...
I am using a student license for Lumion Pro 12.3.1 through my university affiliation.
Good luck!
Arghavan