In order to validate, the HTML would be <a href="/tag:MacBook%20pro" class="p-category">
You could try changing your tag to MacBook%20pro but I think that is going to look weird on your page. In your theme, possibly in a file named “blog_item.html.twig”, look for this code:
{% for tag in page.taxonomy.tag %}
<a href="{{ blog.url|rtrim('/') }}/tag{{ config.system.param_sep }},{{ tag }}">{{ tag }}</a>
{% endfor %}
</span>
and then change the a tag to:
<a href="{{ blog.url|rtrim('/') }}/tag{{ config.system.param_sep }},{{ tag|url_encode }}">{{ tag }}</a>
That should fix your validation issue while leaving the displayed tag as is.