You cannot create the tag
monsters-inc (with a hyphen) when the tag
monstersinc (without a hyphen) already exists. More generally, you cannot create a tag which differs from an existing tag only by the addition or removal of hyphens or a final S.
In most cases, the two tags mean the same thing, so you should keep only one. Pick the one that makes the most sense or follows habitual conventions. For multi-word expressions such as “Monsters, Inc.”, strip the punctuation and use a hyphen to separate words, thus
monsters-inc is the right tag.
When a tag with the wrong spelling has been created, or if the two tags really need to exist (e.g. http ≠ https), ask a moderator: moderators can bypass the restriction, or rename the existing tag. On most sites, you can ping a moderator in the site's main chat room. If you don't do chat or your site's chat room is empty, ask on your site's meta.
The retag privilege lets you retag questions using existing tags. The create tags privilege lets you create new tags. These are different actions from manipulating the tag database, which is a more advanced and more dangerous. There's a privilege to propose tag synonyms, but only moderators have more direct power over the tag database. Issues like this one are rare enough that it's not a big deal to involve a moderator.