This is how some pages look:

 

 

This is how main forum looks:

 

How do I fix?

 

This looks to be an issue with your theme. On the forum main page, you should be able to add a sidebar

10 hours ago, Marc Stridgen said:

This looks to be an issue with your theme. On the forum main page, you should be able to add a sidebar

I switched themes to the default and still cannot do it.

Please could you inform me which page you are looking at there? Looking at your forums page, there are actually side blocks present