Most of your problems are here:
void setup() {
delay(2000)
LEDS.addLeds<APA102, DATA_PIN, CLOCK_PIN, RGB>(LEDS, NUM_LEDS;
Serial.begin(115200); // Teensy/32u4 disregards baud rate; is OK!
No ; after the delay(2000)
No ) at the end of the next line
DATA_PIN is not defined.
Your addLeds line is malformed. I have never used a LED strip with a clock line, but it appears to be missing the SPI_DATA_RATE.
if (bytesBuffered >= HEADERSIZE) has mixed data types. Instead of int, try declaring bytesBuffered thus:
size_t bytesBuffered;