call of overloaded 'String(float&)' is ambiguous

I am using mega 2560 pro. When I try to transfer program to the board I recieve following message.

exit status 1
call of overloaded ‘String(float&)’ is ambiguous

When I tried out code on arduino uno it worked without any issues.
Here is the part of code that triggers error.

 Serial.println(String(floatVariable)+"m/s");

Any help is appreciated.

The String class is best avoided in the Arduino world. It’s a memory hog and can do bad things to your heap memory.

Your line could be:

Serial.print(floatVariable);
Serial.println(“m/s”);

For the same result with less code space and better performance.

Strings cause memory problems and program crashes. Avoid them and instead, use

Serial.print(floatVariable);
Serial.println(" m/s");