[Grammar] I go to a college.

[yi-ing]

My sentence:
It is true that online courses are becoming a common feature of university education.

I can' figure it out why we are not allowed to put article "a" in such examples?

It is true that online courses are becoming a common feature of a university education.

I go to a college.

I used to go a university.

 

[bhaisahab]

You are allowed to use it. It might suggest something slightly different, however. You can also drop it in your other two examples. I think they would be more natural without the article.