Well then you may not be using the correct format.
Make a text box on your layout inside the window.
Make a global variable and call it NUMBER and make it = 1
Make an event: On keyboard key pressed SET TEXT NUMBER
Remove the quotation marks and quotation marks are only used for words and strings of text not variables.
You might also check that your text box is at the top of your layer or it may just be hidden by some sprite above it.
Added: Variable names are case sensitive so make sure you are using the exact variable name as defined. Number, NUMBER and number are not the same variable. You can click on the list and click your exact variable when you set the text which is a good habit to get in.