route file

 <Route name="resourceList" path="/list/:resourceType" handler={AdminList} />
    {/* may need nested route for tab Selections */}
    <Route name="details" path="details" >
      <Route name="companies" path=":resourceType" >
        <Route name="companyId" path=":resourceId" >
          <Route name="resourceDetail" path=":detailTab" handler={AdminDetails} >

            <Route name="createBrand" path="/create/brand" handler={AdminCreateBrand} />
          </Route>
        </Route>
      </Route>
    </Route>

/create/brand is an absolute URL because it starts with /, but in order for its parent routes to work, you need the parent route parameters, which are :detailTab, :resourceId, :resourceType. What do you want the url to look like for createBrand?

Also, you've go two :resourceType segments (from resourceList and companies).