You cannot copy / move a file to your mod folder.

Readme enabled
1. open grass texture FS19 unzip.rar
2. open the readme file by double clicking
3. Open the FS folder, for example: E: \ Farming Simulator 2019 \ data \ foliage or C: \ Program Files (x86) \ Farming Simulator 2019 \ data \ foliage or ... steam ??? \ Farming Simulator 2019 \ data \ foliage, etc ...

! Attention! Important !
4. It is absolutely necessary to backup files that will be exchanged / replaced.
5. Copy textures to the FS folder