You're importing infoboxes from Wikipedia, lots of their infoboxes use Lua (including the one you just imported), so you'll need to copy that all over too.

In general, Wikipedia templates are very complex and have a lot of dependencies. They also tend to have a lot of capabilities that Fandom wikis don't need. In addition, templates with similar names (ex. infobox) are usually created very differently between Wikipedia and most Fandom wikis. As such, it is generally recommended to not copy templates from Wikipedia unless it is absolutely necessary. Instead, where possible, you can try to replicate the appearance but construct your own, simpler template.

In addition to what was said above, if we don't know the specific template there is no way to accurately give you an answer. There are many different ways to make a template work and some have really big dependencies like extensions, Javascript, or Lua. We don't even know if you copied the template correctly.

Ok so the template in question is the 'Infobox military conflict'. Would it be better to just code my own version based on the wikipedia template?

The general layout looks pretty basic. You could get the side-by-side effect by using horizontal groups.