Why doesn't flex work when using BEM technology?

Question

HTML

I use BEM technology, when I'm laying out the header I can't apply
display: flex
flex-direction: column to the element
Elements are still located in the

HTML line

<div class="promo">
        <div class="container">
            <header class="header">
                <a href="#" class="header__logo">
                    <img src="icons/logo.png" alt="logo">
                </a>
                <div class="header__official">
                    Официальный представитель
                    <span>Garmin, Polar, Suunto</span>
                </div>
                <div class="header__contacts">
                    <div class="header__phone">
                        <a href="tel:[phone]">
                            [phone]
                        </a>
                        <button class="button">заказать звонок</button>
                    </div>
                </div>
            </header>
        </div>
    </div>

Here is the header scss file

.header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    &__logo{
        width: 250px;
        img{
            width: 100%;
        }
    }
   &__contacts {
        display: flex;
        flex-direction: column;
        align-items: flex-end;
    }
    &__official {
        color: #fff;
        display: block;
        font-size: 12px;
        font-weight: 500;
        text-align: center;
        span {
            display: block;
            font-size: 18px;
        }
    }
    &__phone {
        display: block;
        color: #fff;
        font-size: 18px;
        font-weight: 700;
        &:hover {
            color: #fff;
            text-decoration: none;
        }
    }
    
}

The phone number should be above the button

Answer 1

It's not clear what flex and BEM have to do with
it

<div class="header__phone">
   <a href="tel:[phone]">
      [phone]
   </a>
   <button class="button">заказать звонок</button>
</div>

Two inline elements will always be located in a line
If you want vertically, you need to override this. For example, wrap the link in a block element (div) or use styles to set display:block to the link and/or button.
Well, or for .header__phone specify flex and vertical direction.

Answer 2

Because you specified a button and a phone in the header__phone class, for which you didn't write any rules.