Call to undefined function mb_strtolower()

I am working on my first opencart project. After I installed OC and the Journal 2 template, my client created a category structure mostly by renaming existing categories in the demo content. The categories are showing in the menu below the logo, but one link refuses to work and takes you to a page with this error
I have compared both the category and link to other functioning links/categories.
I also created a new category and link by the same name (with a 2 added to make it unique) that too gives me the same error.

I pretty much gave it my best but I am unable to solve it and google searches are not helping either. The best I found was some missing or deactivated "mbstring php module ". We have a VPS so I suppose I could install mbstring, but since none of those answers were related to OpenCart I wanted to ask here first and make sure I fix it correctly.

The other reason why I have not yet dealt with mbstring is that all the other categories work fine. What would make a single category generate that error?

I have rebuilt the cart for unrelated reasons and now I get the same error on every single category link.
While previously only one category link out of about 15 gave me this error, now every single one of them does.

The fact that I have been unable to find this error by googling it and there were no responses to my previous post is not encouraging. Yet, I have got the same error in two separate opencart installations. It must be something I am doing wrong and it is surely related to the filters, but I have been unable to even understand the issue.

You need to enable/ install the PHP multibyte extension (mbstring).
Multibyte ext is enabled by default on most server, first think you need to do is to contact your host about this.

Thank you very much for your reply. We have a dedicated server so I'll go ahead and see if I can install it myself.

However, I am puzzled by the fact that this error only affected a single link in the previous website I built, and I was able to get rid of it by just creating a new category and deleting the one that created the error.

in this new site, all the categories links have this problem, but one would think that if a missing component was at the root of it, no category link would have ever worked.

I will go ahead and follow your instructions, but is there a reasonable explanation for this error manifesting only occasionally?

In addition, when I first installed OpenCart with Journal 2, I also installed the demo content and no category failed to work properly. Only after I renamed and created new categories this error showed up.

Thank you for your replay and I'll update this post with the results so it may help others in the future.

Since the error is coming from a commercial theme you'd need to contact the designers for additional details.

Thank you very much. Installing mbstring was easy and the site started working right away.

I owe you my sanity.

Still puzzled as to how some categories worked, but I'll just chalk it up to magic.