CMake error near if: "if given arguments" followed by parantheses, "NOT", "EUQALS" and similar

CMake emits an error in from the following line

if(NOT ($ENV{TnCfg} STREQUAL Linux_Build_Speech))

The error is

CMake Error at CMakeLists.txt:37 (if): if given arguments:

"NOT" "(" "STREQUAL" "Linux_Build_Speech" ")"

Unknown arguments specified

What's the problem? The line is valid code.