why does it say serious error and why can I not add spaces?
Where do you get the warning. And where do you like to use spaces?
Do you get the error when you try and install in Font Book?
Read this: https://glyphsapp.com/tutorials/testing-your-fonts-in-adobe-apps
(Scroll down to ‘No Direct Installation from out of the Adobe Fonts Folder.’)
And then read this on why you shouldn’t use Font Book for testing in the first place: