The carnivore diet is not an optimal way to build muscle/gain weight.
High fat intake promotes satiety which is what we want to avoid if the goal is to build size. We want to maintain our hunger so we can eat more food.
Typically, a moderate protein, high carb, low/moderate fat is the best approach towards building muscle.
Building muscle is a difficult process compared to getting lean. You will see many lean people just walking around day to day, but it’s much more rare to see heavily muscled people. Most people simply don’t have the mental fortitude to eat as much as is required day in and day out to maintain a caloric surplus. Having the right plan that optimizes digestion as well as allows you to eat food you actually enjoy is a big part of it.