Reduce::ratnz
A warning message is generated as a result of the inexact number 6.0 in the input:
Reduce[{x ^ 2 + y ^ 2 == 6.0, x > 0, y > 0}, {x, y}, Reals]The warning message can be avoided by using only exact numbers in the input to Reduce:
Reduce[{x ^ 2 + y ^ 2 == 6, x > 0, y > 0}, {x, y}, Reals]