Imagine a circle of causes. Each part of the circle causes the next part, and so on, until the very first part is arrived at again. Does this avoid the need for a First Cause? Not at all, especially if one excepts even a modest version of the Principle of Sufficient Reason (PSR):
PSR: Everything that exists has an explanation of its existence, either in the necessity of its own nature or in an external cause.
The problem with a circularity of causes is that no explanation for the circle is given. Why is there a circle, rather than no circle at all? Unless one is willing to bite the bullet and say the circle exists by a necessity of its own nature, which is highly unlikely, if not impossible (necessity entails an essence identical with its existence), then on the PSR one is required to reject the possibility of a circularity of causes: the circle has an external cause.