.PageAlerts {
    display: grid;

    grid-template-areas:
        "a"
        "b"
        "c";
    grid-template-rows: auto 1fr 1fr;

    grid-gap: 10px;

    width: 100%;
    height: 100%;
}

.Header {
    grid-area: a;
}

.YourAlerts {
    grid-area: b;
}

.CreateAlert {
    grid-area: c;
}