Data type conversion problem
As said by @Mikael Patel
You need to append .c_str() to u8g2.drawStr(1,10,WiFi.macAddress()); creating the line: u8g2.drawStr(1,10,WiFi.macAddress().c_str());

The problem is that WiFi.macAddress() returns a String not the needed const char* by the drawStr function. c_str converts the String object into a const char*.

Please look at the documentation of the u8g2 library for the drawStr function.

link

And here the documentation for String

link