Making lemon meringue pie and meringue is not stiffing😫 What do I do?

If it's very humid where you are, it's possible it won't stiffen. Meringue can bevery sometimey. I hope you get it to work.

If you mean that you are beating the egg whites and they are not stiffening, you probably dropped some of the yolk into the bowl along with the white--or your bowl had traces of fat. Even the tiniest amount of fat will prevent the egg white from whipping properly. I would throw out the whites you are using, wash the bowl thoroughly with hot water and soap, and separate yolks from whites again, being very careful about not breaking the yolk. Separate your eggs using two small glass bowls. Each time you separate one egg, pour the whites into your CLEAN glass mixing bowl.