Adobe PDF Forms - Calculate Simplified Field Notation

SFN (Simplified Field Notation) doesn't work with field names that have space in them.

You need to escape space character with backslash like this:

Precio\ Compra - Precio\ Venta

Also, your fields are named:

Precio\ CompraRow1 - Precio\ VentaRow1

Instead of using SFN in every field, you can use this as 'Custom calculation script' use it in only one field (any text field is good) it will calculate for all fields:

for( var i=1; i<=33; i++){
 var a = Number(this.getField("Precio CompraRow"+i).valueAsString);
 var b = Number(this.getField("Precio VentaRow"+i).valueAsString);

 if(a !== 0 && b !== 0)
  this.getField("Ganancia  PerdidaRow"+i).value = a-b;
 else
 this.getField("Ganancia  PerdidaRow"+i).value = 0;}

Here is your file with script added:

https://drive.google.com/file/d/1R3smEJsCVv48Xp28dNi2W45oASLwvbGb/view?usp=sharing