There is a known issue, where HardwareSerial (Serial.print() etc) cannot be used after SAVE_TO_SD() is enabled for Arduino UNO (in optimization.h).
As a workaround, the MinimalSerial class is available (part of the bundled SdFat library)
#include <heltec-eink-modules.h>
// Create the replacement serial instance
MinimalSerial s;
void setup() {
// It "should" work pretty much the same as HardwareSerial (UART on Pins 0 and 1)
s.begin(9600);
s.println("Hello, World!");
}
void loop() {
}