Char initialized as concatenation of String +String variables generates error

[JSCSJSCS]

@pylon, Thank you for the clarification. I looked into this further and you are correct. I am using the WiFi.macAddress() from ESP8266WiFi.h.

Arduino does have it's own WiFi.macAddress():

WiFi - Arduino Reference

Arduino's WiFi.macAddress() returns a byte array however (better than a string as you say), while ESP8266WiFi.h returns a string.

I have edited my previous post's code to make this clear.