It should be o.user_id  not doc.user_id because o is variable representing invoice in default odoo.

<div name="user" class="col-xs-2" t-if="o.user_id">
    <strong>Sales Person:</strong>
    <p t-field="o.user_id"/>
</div>

Hi,   

Make sure if the object is doc or o
   just try like this :     

 <div name="reference" class="col-auto mw-100 mb-2" t-if="o.user_id">                  

                  <strong>Sales person:</strong>
                   <p class="m-0" t-field="o.user_id"/>

</div>

Thanks guys, it worked, the 'doc' was definitely the issue.

Thanks a lot everyone, you guys are awesome.