Newbie can't use Solid tool because of "not a solid group"

baiso6.skp (425.0 KB)
Hello everyone! I am a beginner. You guys may not know how valuable your advices mean to me. My question is below:
I want to cut 2 objects: wooden floor and retangular block which is in front of the house. I already made group 2 of them but when I use Solid group, the wooden floor was annouced that “not a solid group”. I am confused, I spent half a day searching for solution but I still have no idea to handle it.
Thank you everyone!

If you are talking about the ‘decking’ you have multiple groups nested within other groups. By exploding down until there is nothing more to explode, you get a single solid group.
Nested groups

Might be good to learn about using Outliner. It gives a clear text version of what you have going on in your model with the wood floor.
Screenshot - 8_12_2022 , 5_20_50 AM

After doing as Box wrote and exploding all the inner groups.

Thank you for your answer, it really works.

you’re right, i think i have to learn more. I’ll try your method later. Thank you for replying!