I had the same problem and resolved this without reinstalling.

If you don’t have a recovery pin setup, you can get it to send to the number of your “First Friend”.
If you don’t have a “First Friend” setup, you can set this number in your Avast account on the website.

Disable anti-theft from being a device administrator and recover your PIN

  1. On you phone ,,
  2. uncheck the name of the app that anti-theft installed as (is an android robot icon), if you have the right one, it should say something like "This administrator is active and allows the app to: Erase all data, Change the screen-unlock password, lock the screen, etc.
  3. Select the button
  4. It will ask for your Avast pin (which you don’t know), BUT it also lets you choose to send a recovery pin, it will send you your “First Friend” phone number and will be active for 30 mins. If you don’t have a First Friend number set, follow the second set of steps below; “Set up a First Friend number via the web.”
  5. Start Avast, enter the recovery pin (which you got from step 4 above)
  6. Click <avast! PIN>
  7. Change the Avast PIN
  8. Set the phone number for PIN recovery
  9. Follow steps 1. & 2. above to re-enable anti-theft as a device administrator

If you don’t have a first friend setup…

Set up a “First Friend” number via the web.

  1. Login to https://my.avast.com/en-gb
  2. Select your device
  3. Choose and the tab
  4. Down the bottom is a button, select it.
  5. On the tab, set the phone number of your “First Friend” and click OK
  6. Follow the first set of steps above to “Disable anti-theft from being a device administrator and recover your PIN”

Regards

-KENT