Could not find method cppFlags() arguments() for arguments
There are two entirely different blocks in build.gradle that are named externalNativeBuild (thanks, Google).
One is under android top-level block, and it specifies the path to native build script, cmake or ndkBuild (but not both).
The other can be used to specify, from gradle, some parameters for the native build. This block is usually a child of the defaultConfig block, but can also appear for flavors (see an example).
These externalNativeBuild blocks can also have cmake or ndkBuild children, but only the child that matches the path setting (see above) is relevant, the other is silently ignored.
The bottom line, split your block in two, and put each one in correct place in build.gradle hierarchy.