How do I draw a horizontal line between two circles with CSS?

You can add a new element and position it between the two circles:

#status-buttons a {
    color: black;
    display: inline-block;
    font-size: 17px;
    font-weight: normal;
    margin-right: 0;
    text-align: center;
    text-transform: uppercase;
    min-width: 150px;
    text-decoration: none;
}

#status-buttons a:hover {
  text-decoration: none;
}
    
#status-buttons a.active span {
    color: white;
    background: #ACCF5B;
    box-shadow: rgba(0, 0, 0, 0.792157) 3px 3px 3px 0;
}

#status-buttons span {
    color: white;
    background: #22bacb;
    display: block;
    height: 45px;
    margin: 0 auto 10px;
    padding-top: 20px;
    width: 60px;
    border-radius: 50%;
    box-shadow: rgba(0, 0, 0, 0.792157) 3px 3px 3px 0;
}

#line {
  position: absolute;
  top: 42px;
  left: 112px;
  width: 96px;
  height: 5px;
  background: #ACCF5B;
}
<div id="status-buttons" class="text-center">
                <a href="#/form/regalo" class="active"><span>1</span> Step 1</a>
                <div id="line">
                </div>
                <a href="#/form/tusdatos"><span>2</span> Step 2</a>
            </div>

You can use a pseudo-element to insert an absolutely-positioned border:

#status-buttons {
  position: relative;          /* 1 */
  display: inline-block;       /* 2 */
}
#status-buttons::after {       /* 3 */
  content: "";
  position: absolute;
  width: 50%;
  z-index: -1;                 /* 4 */
  top: 35%;
  left: 25%;
  border: 3px solid #ACCF5B;
}
#status-buttons a {
  color: black;
  display: inline-block;
  font-size: 17px;
  font-weight: normal;
  margin-right: 0;
  text-align: center;
  text-transform: uppercase;
  min-width: 150px;
  text-decoration: none;
}
#status-buttons a:hover {
  text-decoration: none;
}
#status-buttons a.active span {
  color: white;
  background: #ACCF5B;
  box-shadow: rgba(0, 0, 0, 0.792157) 3px 3px 3px 0;
}
#status-buttons span {
  color: white;
  background: #22bacb;
  display: block;
  height: 45px;
  margin: 0 auto 10px;
  padding-top: 20px;
  width: 60px;
  border-radius: 50%;
  box-shadow: rgba(0, 0, 0, 0.792157) 3px 3px 3px 0;
}
<div id="status-buttons" class="text-center">
  <a href="#/form/regalo" class="active"><span>1</span> Step 1</a>
  <a href="#/form/tusdatos"><span>2</span> Step 2</a>
</div>

Notes:

  1. Establish nearest positioned ancestor for absolute positioning.
  2. Make container consume only the width necessary.
  3. Insert pseudo element
  4. Ensure that any horizontal line overlap doesn't appear above circles