If you forgot to file the PI-1206 Homeschool Report for any year, your child is/was subject to the truancy process for the time period in which you were not in compliance with compulsory attendance laws.
There is no way to file retroactively. You are 100% responsible for compliance with the law. If you did not file for any year, you do not have proof that you were in compliance with Wisconsin homeschooling law for that year. There is nothing you or WHPA can do to fix this.