Modbus Protocol Compile Error
Not sure where you got the library from, but if it's from this github repo, you're using it wrong.

The function prototype for modbus_configure is:

void modbus_configure(long baud, byte _slaveID, byte _TxEnablePin, unsigned int _holdingRegsSize, unsigned char _lowLatency);


The example file shows how to use it correctly

modbus_configure(115200, 1, 2, TOTAL_REGS_SIZE);