There could be a couple of reasons for this.
1) You have some kind of malware on the system that is preventing you accessing security programs, control panel etc (such things exist, and are often a reason users turn to a different AV),
2) You actually aren't running from an admin account,
Also possible, quite likely really, the remnants of another security program are interfering with Avast.
Please post the answers to the questions above posed by Mr A, and also a brief general description of why you downloaded Avast, and what you were using before.
If you know you are running from an admin account, post that too. If you don't know what sort of account you are running from, we can help you find that out.
This problem is fairly likely to be easy to solve, unless it's caused by malware, then it might be a little more involved.
Welcome to the forum.
