Found it!
.html
<main>
<div class="row" align="center">
<div class="col ">
<div class="title-box">
<h1> ATVs, UTVs, Garden and Compact Tractors</h1>
</div>
<div>
<div class ="col col-md-one-half col-full"><img src="Images/Pull-Drag.jpg"></div>
<div class="col col-md-one-half col-full"><img src="Images/Pull-Dragstr.jpg"></div>
<div class="col col-md-one-half col-full"><img src="Images/Pull-Rake.jpg"></div>
<div class="col col-md-one-half col-full"><img src="Images/Pull-EquiG-QD.jpg"></div>
</div>
<div class="row" align="center">
<div class="col ">
<div class="title-box">
<h1> 3-Point Hitch and Large Tractors</h1>
</div>
<div class ="col col-md-one-half col-full"><img src="Images/Hitch-Max.jpg"></div>
<div class="col col-md-one-half col-full"><img src="Images/Hitch-Shark.jpg"></div>
<div class="col col-md-one-half col-full"><img src="Images/Hitch-EG-TR.jpg"></div>
</div>
<div class="row" align="center">
<div class="col col-md-one-half col-full"><img src="Images/ind-jump.jpg"></div>
<div class="col col-md-one-half col-full"><img src="Images/ind-turn.jpg"></div>
</div>
</main>
And this is how it looks. Sorry it took so long. My computer is painfully slow.

display: grid; isnt supported in some browsers yet, its still behind a flag.
Are you using display: grid; as an experiment for laying out your page or are you using it because you don't know what it is but just following a tutorial?
.col-full {
width: 20%;
display:grid ;
}