Impossible to invoke a method ("limit") on an array
craft.entries now returns an ElementQuery. When you add the .all() you're telling Craft to execute the query, which returns an array of Elements.

That means that solutions is now a simple array, and won't have methods like .limit(1) that can be executed on it. It looks like you just want to use the first item in the solutions array, so try changing this:

      {% nav solution in solutions.limit(1) %}
           &lt;a class="nav-link" href="{{ solution.url }}"&gt;solutions&lt;span class="sr-only"&gt;(current)&lt;/span&gt; &lt;i class="icon-downarrow"&gt;&lt;/i&gt; &lt;/a&gt;
      {% endnav %}


To this:

       &lt;a class="nav-link" href="{{ solutions[0].url }}"&gt;solutions&lt;span class="sr-only"&gt;(current)&lt;/span&gt; &lt;i class="icon-downarrow"&gt;&lt;/i&gt; &lt;/a&gt;


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