Both spellings are correct English -- which one you should use depends on which convention your institution follows, not which one is objectively right.
The short answer
"Defence" is British/Commonwealth English. "Defense" is American English. Nigerian English generally follows British conventions, so "defence" is the more commonly expected spelling in formal academic writing at most Nigerian institutions -- but "defense" isn't wrong, it's simply American.
What to actually check
Your institution's style guide, if one exists, or how your own department's official documents spell it. When there's no explicit guidance, default to British spelling ("defence," along with "colour," "organisation," and similar) since that's the broader convention in Nigerian academic English.
The one rule that actually matters: consistency
Whichever spelling you choose, use it throughout your entire document. Switching between "defense" and "defence" within the same report -- easy to do accidentally if you're referencing sources or templates using both conventions -- reads as careless regardless of which spelling is technically preferred for your context.
Other British vs. American spellings to watch for
While you're checking this, the same pattern applies to other words: "organisation" (not "organization"), "colour" (not "color"), "analyse" (not "analyze"), "programme" (not "program," except when referring to software) -- all more standard in Nigerian academic English.
Where UniDraft fits in
UniDraft applies consistent spelling conventions throughout your generated document automatically, so this kind of small but noticeable inconsistency isn't something you need to manually proofread for.