TMC26XMotorTester:44: error: 'TMC26XStepper' does not name a type
Arduino libraries are normally written as a C++ Class.  When you start a sketch you normally tell the compiler where to find that Class and to Instantiate that class.  If the compiler does not find the Class and you attempt to Instantiate it, the compiler will telly you it does not know what to do.  That is, it will tell you the thing you are using (TMC26XStepper) does not describe anything or is not a Type.

Study the structure of Arduino libraries to make sure what you have installed agrees with the common Arduino library.

Also, newer version of the Arduino IDE can download and unpack libraries automatically given both the IDE and the library are setup to do this.