You cannot put an expression statement outside a function. Only the variable declarations can be placed outside functions (and those variables are called globals/global variables).
Try the following:
float x=10; float y=50; float z;
void setup() {
// put your setup code here, to run once:
z=x*y;
}
void loop() {
// put your main code here, to run repeatedly:
}
Update
What even is easier and also works (thanks to Ruslan and Juraj):
float x=10; float y=50; float z=x*y;
void setup() {
// put your setup code here, to run once:
}
void loop() {
// put your main code here, to run repeatedly:
}