You missed "float" when you declared fToC(float degreesF), and it should be f"T"oC not f"t"oC when you invoked it.
#include <iostream>
using namespace std;
//float is missed below
float fToC(float degreesF){
float degreesC = ((5.0/9.0)*(degreesF-32.0));
return degreesC;
}
int main(){
float fahrenheit;
float centigrade;
cout << "Enter a Fahrenheit temperature:" << endl;
cin >> fahrenheit;
centigrade = fToC(fahrenheit); //should be "T" here, not "t"
cout << fahrenheit << "F is " << centigrade << "C";
return 0;
}