There are various solutions available
Approach 1
I don't believe using row provides any benefit in this scenario, so feel free to remove it unless there are col- elements within your footer
<footer class="p-1 mt-3 bg-primary text-white">
<p class="text-center">footer</p>
</footer>
Approach 2
If you prefer to keep the row, simply include the class justify-content-center to your row element
<footer class="row p-1 mt-3 bg-primary text-white justify-content-center">
<p class="text-center">footer</p>
</footer>
Approach 3
Add a new element and assign the class col-12 to it
<footer class="row p-1 mt-3 bg-primary text-white ">
<div class="col-12">
<p class="text-center">footer</p>
</div>
</footer>