Found this, which I posted years ago; should solve your problem:
Fix for negative/bugged character armor rating:
This fix requires the console, so it's only useable on the PC.
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People occasionally post for help because their overall armor rating has a negative (or otherwise incorrect) value, and I don't remember seeing any solutions posted.
If fixes for this are already available, my apologies ; I haven't read the General or Hints, etc. forums regularly for a while. A search just found people with the problem; I didn't find any solutions.
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To avoid confusion:
This is NOT about damaged armor skills (from equipping Fury, for example) or damaged armor. Solutions to those problems have been posted repeatedly already.
This is for the situation where somebody's armor skills are at their normal values, they don't have any armor equipped, and the character isn't affected by any spells that would modify their armor rating or armor skills, but their armor rating is negative (or different from what it should be based on their Unarmored skill level).
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The fix:
First, backup your saved game. This procedure shouldn't cause any problems, but make a backup just in case anyway.
1. Make sure your character's armor skills aren't damaged; restore at a shrine or altar if necessary.
2. Make sure your character isn't affected by any spells, Constant Effect items, etc. that modify overall armor rating (such as spells with a Shield effect) or armor skills.
3. Remove all your character's armor.
4. Once your character's armor is removed, calculate their correct overall armor rating based on their Unarmored skill. If your Unarmored skill is roughly 12 or above, you should have a non-zero armor rating even with no armor equipped.
The formula is "Unarmored Armor Rating = ((Unarmored Skill)**2) * .0065".
Or "unarmored skill X unarmored skill X .0065", which is another way of writing the same thing.
5. If the overall armor rating shown below your character's portrait is higher or lower than calculated in step #3, then you can use 'SetArmorBonus' from the console to reset it to the correct value.
5a. First, double check the first three steps.
5b. Hit the '~' key to open up the console window.
5c. The format for the command is as follows; substitute the correct number for the '20' in my examples:
player->SetArmorBonus 20 (or whatever number)
Type that in and hit 'Return' to enter the line, then hit the '`' key to close the console window again.
5d. You won't see the new values immediately; save and reload your game so the changes will take effect.
(Or maybe just moving from an interior cell to an exterior cell (or vice versa) will cause it to update; don't remember if that works for armor rating.)
6. Once you've saved and reloaded, double check the overall armor rating shown below your character's portrait to make sure it's now correct: it should match the value calculated in step #4.
If it's now correct, great! If not, try again.