I've found some other errors on Google Webmaster tools from 4 of my blog posts
when I went to check them the links give an error "0-Class 'CBuser' Not Found" and they show without any CSS class.

I do not know what I have wrong in the setup.
I have a menu called Blog Menu where I have the posts as single posts, regardless 6 of them work fine, but 4 of them have that error.

https://insidepiano.com/es/lecciones-de-piano-acordes-de-paso
https://insidepiano.com/es/lecciones-de-piano-chord-melody
https://insidepiano.com/es/lecciones-de-piano-composición-multi-tonica
https://insidepiano.com/es/lecciones-de-piano-acordes-abiertos-de-tríada-mayor

Can you please help?

Thank you