Line 6. Your method signature needs types for the parameters. Either add "double " before both parameters, or drop the parameters altogether since you're not using them anyway.
Line 11. else with a guard is not allowed. Change it into "else if (this.saldo < this.valor)".
Line 9, 13. Code directly after a return will never ever be executed, so it's simply not allowed. Move the return statement to after the print-outs.
Line 15. What will be returned if this.saldo == this.valor? You'll need to return something there as well.
Note that the compiler isn't smart enough to see that if you change line 11 to "else if (this.saldo <= this.valor)" you actually have covered all cases. It will still think that it's possible that there is a third option, so you'll need a return statement.
The answer to the question you asked is "Because your code doesn't follow proper syntax".
People here will bend over backwards to help you (as Bear and Rob have already done). Your job is to make it as easy for them to help you as possible. That includes posting the error messages your got, what SPECIFICALLY you don't understand, and as much other information as you can.
Generally speaking, posting something along the lines of "This doesn't work" will be met with silence.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors