Choose a measure in the locked system and use Utilities > Unlock Systems (CTRL-SHIFT-L).

… There's a system break in my score that I want to get rid of and just have it reappear as one continuous system …

From your description it sounds like you have a Coda System = two systems next to each other, with some horizontal space between them.

My guess is that the “break” was created by the plug-in Create Coda System.

To get rid of the “break”, use the Page Layout menu command Redefine Pages.

NB:

When you Redefine Pages - and whenever Finale creates new pages - the new pages are formatted according to the settings in

Document menu > Page Format > Score…

Before you Redefine Pages, take a look at the settings in Page Format for Score - and edit/correct the settings if they are not as you need them.