According to the documentation for dijit/layout/BorderContainer, each child of the BorderContainer must have a "region" attribute that tells where it's located on the page. For example:
<div data-dojo-type="dijit/layout/BorderContainer" data-dojo-props="gutters:true, liveSplitters:true" id="layoutPriorityBorderContainer">
<div data-dojo-type="dijit/layout/ContentPane" data-dojo-props="splitter:true, region:'left'" style="width: 100px;">Left</div>
<div data-dojo-type="dijit/layout/ContentPane" data-dojo-props="splitter:true, region:'center'" style="width: 100px;">Center</div>
<div data-dojo-type="dijit/layout/ContentPane" data-dojo-props="splitter:true, region:'right'">Right</div>
</div>
OR:
<div data-dojo-type="dijit/layout/BorderContainer" data-dojo-props="design:'sidebar', gutters:true, liveSplitters:true" id="layoutPriorityBorderContainer">
<div data-dojo-type="dijit/layout/ContentPane" data-dojo-props="splitter:true, region:'leading', layoutPriority:1" style="width: 100px;">Left #1</div>
<div data-dojo-type="dijit/layout/ContentPane" data-dojo-props="splitter:true, region:'leading', layoutPriority:2" style="width: 100px;">Left #2</div>
<div data-dojo-type="dijit/layout/ContentPane" data-dojo-props="splitter:true, region:'center'">Hi, I'm center</div>
</div>
Make sure each child of the BorderContainer is a dijit with a region assigned.