I have tried to place a youtube video in an article and it does not allow me.
I am using purity iii template in joomla 4.
In other templates I have already done it before but in this one I cannot do it.
Do I have to configure something in particular?
Purity III template is used on frontend only, adding video or other content inside an article must belong to default Joomla. You can switch to Cassiopeia - default Joomla 4 template and see how it goes.