I am getting this error frequently too. Example click tick box next to quest in Guides panel. I have repaired and reinstalled losing all my saved variables. I have tested it with no other add ons and still continually getting this error. Installed 3.027 but it was happening with previous versions. I had hoped 3.026 would sort it. Obviously not.
Message: Usage: self:SetNormalTexture(asset)
Time: Tue Feb 28 12:12:30 2023
Count: 1
Stack: Usage: self:SetNormalTexture(asset)
[string "=[C]"]: in function `SetNormalTexture'
[string "@Interface/AddOns/DugisGuideViewerZ/SmallFrame.classic.common.lua"]:735: in function <...dOns/DugisGuideViewerZ/SmallFrame.classic.common.lua:707>
[string "@Interface/AddOns/DugisGuideViewerZ/SmallFrame.classic.common.lua"]:1163: in function `PopulateSmallFrame'
[string "@Interface/AddOns/DugisGuideViewerZ/SmallFrame.classic.common.lua"]:1756: in function `UpdateSmallFrame'
[string "@Interface/AddOns/DugisGuideViewerZ/DugisGuideViewer.lua"]:5199: in function `UpdateCompletionVisuals'
[string "@Interface/AddOns/DugisGuideViewerZ/Guides.classic.common.lua"]:3507: in function `MoveToNextQuest'
[string "@Interface/AddOns/DugisGuideViewerZ/SmallFrame.classic.common.lua"]:1210: in function <...dOns/DugisGuideViewerZ/SmallFrame.classic.common.lua:1208>
[string "@Interface/AddOns/DugisGuideViewerZ/LuaUtils.lua"]:688: in function <Interface/AddOns/DugisGuideViewerZ/LuaUtils.lua:686>
Locals: (*temporary) = "Usage: self:SetNormalTexture(asset)"