[Vuejs]-Cannot read property of undefined when trying to access a method from another method

In Javascript this acts differently than other languages like Java.
Please read this mdn page.

When you use this inside forEach, it refers to forEach context.

To answer your question, you can store this in a variable:

var self = this;
restrictedDishes.forEach(function(item){
    specifications.push({
        type: "Dish",
        item: item.name,
        meals: self.getStringMeals(item.pivot.breakfast,item.pivot.snack1,item.pivot.lunch,item.pivot.snack2,item.pivot.dinner)
    })});

Or use fat arrow syntax:

restrictedIngredients.forEach((obj) => {
   specifications.push({
       type: "Ingredient",
       item: item.name,
       meals: this.getStringMeals(item.pivot.breakfast,item.pivot.snack1,item.pivot.lunch,item.pivot.snack2,item.pivot.dinner)
       })});