[Bruno1337]

In KDFF GUI & BLENDER object looks like --> https://imgur.com/a/pRCcAUJ

But in game object looks like --> https://imgur.com/a/zPKE5eY

What I need to do? How to fix this problem with walls?

[REYO]

Make sure the floor plane you're using is double-sided, then export and try again.

[ATomas]

You must show normals. double-sided textures in samp doesnt work.

http://www.img.tpx.cz/uploads/agvaesgcergfcecgw.png
normals must be direction out

or youcan flip all selected normals

http://www.img.tpx.cz/uploads/evgrtvtrvrtvhrt.png

[Yaozu]

The object looks a bit dark you might want to apply the day and night colours to your object by using a Radiosity solution or by using KDFF.

[Bruno1337]

Thank you Tomas, thank you all! https://imgur.com/a/l3n6yQP