Plugin SpriteFont+ (SpriteFontPlus) by Chris Kent, please!
You don't need that, the Construct 3 SpriteFont official plugin is a duplicate of the SpriteFont+ plugin.
If you already have a Construct 2 Project that has SpriteFont+ and can't be converted into Construct 3 for that reason, you can replace the id SpriteFontPlus into Spritefont2 from the #ProjectName.caproj XML file. Then that should solve it.
Although, backup before trying anything. To make sure you don't lose anything in a case you make mistakes and need to start all over.