What does it mean by the error “10 is not a valid variable”?

I wrote a very basic test function in Mathematica (I am the absolute beginner)

Fun1[x_] := Module[ {y, z, t}, y = Min[x + 10000, 500]; z = y + 24; t = y + z; t ] 

However, when I try to solve for the root Solve[Fun1[x] == 0, x], I was never able to get the expected value, but this error occurred "… Solve::ivar: 10 is not a valid variable".

what does it mean? and how to overcome this?

Many thanks Tien