Category is too long
Webmaster site category exceeded 120 characters after trim.
Why this happens
Paste of a long description into the category field. Categories are labels, not full site blurbs.
Diagnose and fix
1. Shorten to ≤ 120 characters (or clear the field if optional).
2. Put longer text in the site title/description fields if those exist and allow it.
3. Retry add website.
4. See Category contains invalid characters if shorten still fails.