When I make buttercream (cold-process "American" BC, from the recipe that's been on the back of the C&H powdered sugar box since before most of us were born), I use plain store-brand salted butter, which, in the U.S., is invariably churned from sweet cream.
If you're using unsalted, either try salted butter, or add salt yourself.
If you're using salted with a recipe that adds salt, try leaving out the additional salt.
If you're using butter made from fermented cream, try using butter made from sweet cream.
An odd observation I made, some time ago: the margarine we use at home, for both cooking and table use ("I can't believe . . ."), is actually more expensive than the store-brand butter we use for (again) both cooking and table use. (And despite the name of the product, I can tell almost instantly if somebody grabbed the wrong stick [we serve butter in a glass butter dish, and margarine in a plastic one], usually before it gets within a foot of my mouth.) And as I recall, the vegan, dairy free margarine I bought some time ago for a single batch on the "wood type cookie" project is even more expensive.
(Incidentally, for sauteing the onions and celery for turkey dressing, I've started using a 50/50 mix of unsalted butter and salted margarine. And for sauteing Rice-a-Roni, I've taken to using a 50/50 mix of salted butter and Canola oil.)