What Avast product did you try for free, I think that the Avast Premium Security trial is 30 days, at the end of which the auto payment mandate (you complete) is actioned. Unless you have terminated the trial/subscription and the auto payment mandate, this will be actioned.
You should have got an email prior to the payment as far as I can remember, the most common reason for not receiving this is having made a type in your linked email address.
Refund request -
https://support.avast.com/en-ww/contact/sales_refund. You will be asked for information, e.g. your email address used and transaction ID, if you don't know this enter 00000000 and give as much information to help identify your subscription.
