Running Software (please complete the following information):

• Game patch 8.0.1 (27842)
• Addon version 8.0.6

On log-in. I have deleted and reinstalled with the same errors.

Error Logs

3x PetTracker\Features\BattleListener.lua:39: bad argument #1 to 'pairs' (table expected, got nil)
[C]: in function pairs' PetTracker\Features\BattleListener.lua:39: in function Upgrade'
PetTracker\Features\BattleListener.lua:30: in function ?' PetTracker\PetTracker-8.0.6.lua:40: in function ForAllModules'
PetTracker\PetTracker-8.0.6.lua:54: in function `?'
PetTracker\PetTracker-8.0.6.lua:21: in function <PetTracker\PetTracker.lua:21>

Locals:
(*temporary) = nil
(*temporary) = "table expected, got nil"
= defined =[C]:-1

Try following those steps :

• Exit World of Warcraft
• Delete <wow folder>\WTF\Account\<your account>\SavedVariables\PetTracker.lua
• Delete <wow folder>\WTF\Account\<your account>\SavedVariables\PetTracker.lua.bak

This will reset PetTracker's configuration. I had exactly the same problem as you and this fixed the issue. Hope this helps.