Self-published sources are generally not considered reliable: i.e. anyone can't just write a web page or a book and start quoting themselves on Wikipedia. That's clearly objectionable!

Recognized domain experts who are routinely published in respectable journals is one of the main exceptions.

Some relevant policy: https://en.m.wikipedia.org/wiki/Wikipedia:Verifiability#Self...

You can use primary sources. There are rules about them and a preference for secondary sources, but no blanket ban.