If you’re running on a deficit it will inhibit muscle growth yes because your body won’t have the materials it needs to build new muscle as quickly (this could also be the case if you weren’t on a deficit but eat a garbage diet) but that doesn’t mean you won’t make gains at all. Whoever’s telling you this metabolism stuff probably doesn’t know what they’re talking about and it shouldn’t be what you’re focusing on. Start with lowering your calorie intake and go from there. I’d suggest getting a calorie tracking app to help you figure out a diet plan that keeps your carbs/protein/fat in order and do moderate workouts while you’re dieting. I’ve used myfitnesspal in the past but I’m sure there are other options.