Buildin variables doesn't exist?

I am trying to run my code but it says that variables like ‘size’, ‘background’, ‘mousePressed’, ‘mouseX’ and so on doesn’t exist. Basically all the builtin defined variables. There just comes this red line underneath, and it says “The variable ‘mouseX’ does not exist”, or something like that. How can this be when the variables that I have defines myself like ''akser" works fine? (I don’t know if they work, because I have not yet got to run the code, but it doesn’t get a red line under it).

Does this happen when you create a new empty sketch and do something like void setup(){size(1,1);} ?
In other words, is this specific to this exact project, or all of them?

If it’s specific to this project, could you please post the code here so that I could help figure it out?