If you've just added a new category in WordPress, you'll likely need to refresh the category options that display when setting up Category Targeting!
Step 1: Head to Grow Publisher Portal > Subscribe > Forms and click Manage Form on a Spotlight or Pop Up Subscribe form
Step 2: Scroll down to the Targeted Categories section and push the Refresh Categories button:
And that's it! Be sure to do a hard refresh on the page and you should see the additional categories.
Note: If you're still having trouble seeing categories after refreshing your category options, check your security plugin(s). If your REST API is being restricted by your security plugin(s), it can prevent Grow from accessing your category data.