Making instant pot lemon cake. Calls for 1 1/2 cups of all purpose flour ALL I have is cake flour.
You should use the same amount of flour, 1 1/2 cups.
As others have pointed out in the comments, cake flour and all purpose flour are not somehow convertible to each other by amount. They have different properties, and produce different results in baked goods. So there is not a perfect substitution where you just have to change the volume, like you could have done with e.g. powdered sugar instead of crystal sugar.
At the same time, there is a very good chance that your cake will turn out to be good if you make it with cake flour. First, cake flour generally makes good cakes - very few recipes require all purpose flour, they are more likely made with it because it is ubiquitous. Second, cake batters are rather forgiving to changes in gluten content (which is the main difference between cake and ap flour). Third, you are obviously not trying to get Michelin stars with an instant pot cake. At home, people are less critical of the cake they are eating, because they don't have the overhoned expectations of professional bakers. Fourth, if the cake is not that great, it will probably be due to it being an instant pot cake (which are never as good as the ones from the oven) than the change in flour.
So, in short, use the flour in a 1:1 substitution and see what happens. It will in most likelihood be a cake you enjoy.