Answers for "center an element horizontally on div css bootstrap"

center div horizontally and vertically

.parent {
  position: relative;
}
.child {
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

bootstrap center button horizontally

<button type="submit" class="btn btn-primary mx-auto d-block">Submit</button>