git: fatal: ‘–ours/–theirs’ cannot be used with switching branches

Using git checkout with --ours or --theirs expects at least one argument: the path(s) of the files / directories to checkout.

As the manual says:

When checking out paths from the index, check out stage #2 (ours) or #3 (theirs) for unmerged paths.

So:

git checkout --ours <path(s)>