Message: Couldn't open Interface\AddOns\Skillet\Libs\AceAddon-3.0\AceAddon-3.0.xml
Time: Thu Feb 18 03:34:06 2021
Count: 3

and...

Message: Error loading Interface\AddOns\Skillet\Libs\LibWindow-1.1\LibWindow-1.1.lua
Time: Thu Feb 18 03:34:06 2021
Count: 3

Upon inspection of folder structure, detected that there is no AceAddon-3.0 folder

same problem here as well

Just have to find some other addon with AceAddon-3.0\AceAddon-3.0.xml and copy/past into skillet for 1st lua error
For the second, just move LibWindow-1.1.lua from Interface\AddOns\Skillet\Libs\LibWindow-1.1\LibWindow-1.1\LibWindow-1.1.lua ito the folder Interface\AddOns\Skillet\Libs\LibWindow-1.1
and after a reload, all the error are gone.
at least the errors are gone for me after that folder replacement. (there is a mistake with double named folder for error 2)

Same errors. Read over Cudyhouse and figured out what he was saying. Made those suggested changes but now I get:
1x [ADDON_ACTION_FORBIDDEN] AddOn 'Skillet' tried to call the protected function 'UNKNOWN()'.
[string "@!BugGrabber\BugGrabber.lua"]:519: in function <!BugGrabber\BugGrabber.lua:519>
[string "=[C]"]: ?
[string "=[C]"]: in function CastSpellByName' [string "@Skillet\Skillet-4.21.1.lua"]:1167: in function ChangeTradeSkill'
[string "@Skillet\Skillet-4.21.1.lua"]:1219: in function SetTradeSkill' [string "@Skillet\UI\MainFrame.lua"]:626: in function TradeButton_OnClick'
[string ":OnClick"]:2: in function <[string ":OnClick"]:1>

Skillet-4.22 should fix this issue. The automated repository conversion from SVN to GIT missed some stuff.

@Excelis, you are reporting two different issues. See issue #1 for the second issue (hint: It's Blizzard's fault, not Skillet's)