Seems to compile

ampel-firmware/co2_sensor.cpp

@@ -10,6 +10,8 @@
 // https://github.com/sparkfun/SparkFun_SCD30_Arduino_Library
 #include "src/lib/SparkFun_SCD30_Arduino_Library/src/SparkFun_SCD30_Arduino_Library.h"  // From: http://librarymanager/All#SparkFun_SCD30
 
+#include "s8_uart.h" // Not used yet
+
 namespace config {
   const uint16_t measurement_timestep_bootup = 5; // [s] Measurement timestep during acclimatization.
   const uint8_t max_deviation_during_bootup = 20; // [%]

platformio.ini

@@ -27,6 +27,7 @@ framework = arduino
 monitor_speed = 115200
 lib_deps =
     MCCI LoRaWAN LMIC library
+    S8_UART
 build_flags =
     -D ARDUINO_LMIC_PROJECT_CONFIG_H_SUPPRESS
     -D CFG_eu868=1