Your second set of columns need to be a child of a row div, and should not total more than 12 (yours total 14). So make it like this:
<div class="container">
<div class="row">
<div class="col-sm-10 mx-auto">
<div class="row">
<div class="col-sm-2">
<button class="btn-dark" (click)="previousPage()">Previous</button>
</div>
<div class="col-sm-10 text-right">
<button class="btn-dark" (click)="nextPage()">Next</button>
</div>
</div>
</div>
</div>
</div>
Codeply example
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