"Build" and "Build and Run" appears disabled when both Mono and IL2CPP Linux build supports are installed and IL2CPP is set as Scripting Backend while building project.
Also, "Currently selected scripting backend (IL2CPP) is not installed" warning appears in the "Build Settings" window.
However, when the project can be built when Scripting backend is set to Mono for the same project.

Steps to repro:
1. Installed Linux build support (IL2CPP) and Linux build support (Mono)
2. Create a New Project
3. Add Toolchain Win Linux x64 Package
3. File > Build Settings > Build Settings windows > Target Platform > Linux
4. Edit > Player Settings > Player > Scripting backend > IL2CPP

Actual Result:
"Build" and "Build And Run" buttons appear disabled and "Currently selected scripting backend (IL2CPP) is not installed" warning appears in the "Build Settings" window.

Expected Result:
The project is built successfully with IL2CPP Scripting Backend.

Note: Linux build support (IL2CPP) for 2020.2 is introduced in 2020.2.0a19, and for 2020.1 is introduced 2020.1.5f1.

Reproducible in:
2020.2.0b2, 2020.2.0a19, 2020.1.5f1

Workaround
When only Linux build support (IL2CPP) is installed then the project can be built.

Environment:
Windows 10 & macOS 10.14