Describe the bug
If you have ElvUI enabled then you get this errors

Error Log
1744x C stack overflow
[string "=[C]"]: ?
[string "=[C]"]: in function SetVertexColor' [string "@Blizzard_TradeSkillUI/Wrath/Blizzard_TradeSkillUI.lua"]:336: in function <...lizzard_TradeSkillUI/Wrath/Blizzard_TradeSkillUI.lua:313> [string "=[C]"]: ? [string "=[C]"]: ? [string "=[C]"]: in function TradeSkillFrame_SetSelection'
[string "@Blizzard_TradeSkillUI/Wrath/Blizzard_TradeSkillUI.lua"]:137: in function <...lizzard_TradeSkillUI/Wrath/Blizzard_TradeSkillUI.lua:127>
[string "=[C]"]: in function SetTradeSkillSubClassFilter' [string "@Blizzard_TradeSkillUI/Wrath/Blizzard_TradeSkillUI.lua"]:50: in function TradeSkillFrame_SetupSubClassDropdown'
[string "@Blizzard_TradeSkillUI/Wrath/Blizzard_TradeSkillUI.lua"]:160: in function TradeSkillFrame_Update' [string "@Blizzard_TradeSkillUI/Wrath/Blizzard_TradeSkillUI.lua"]:141: in function <...lizzard_TradeSkillUI/Wrath/Blizzard_TradeSkillUI.lua:127> [string "=[C]"]: in function SetTradeSkillSubClassFilter'
...
[string "=[C]"]: in function Show' [string "@Blizzard_UIParentPanelManager/Classic/UIParentPanelManager_Shared.lua"]:603: in function SetUIPanel'
[string "@Blizzard_UIParentPanelManager/Classic/UIParentPanelManager_Shared.lua"]:512: in function ShowUIPanel' [string "@Blizzard_UIParentPanelManager/Classic/UIParentPanelManager_Shared.lua"]:316: in function <...PanelManager/Classic/UIParentPanelManager_Shared.lua:312> [string "=[C]"]: in function SetAttribute'
[string "@Blizzard_UIParentPanelManager/Classic/UIParentPanelManager_Shared.lua"]:1091: in function <...PanelManager/Classic/UIParentPanelManager_Shared.lua:1070>
[string "=[C]"]: in function ShowUIPanel' [string "@Blizzard_UIParent/Cata/UIParent.lua"]:1143: in function <Blizzard_UIParent/Cata/UIParent.lua:646> [string "=[C]"]: in function CastSpell'
[string "@Blizzard_UIPanels_Game/Cata/SpellBookFrame.lua"]:402: in function <...ddOns/Blizzard_UIPanels_Game/Cata/SpellBookFrame.lua:383>

Locals:

Versions (please complete the following information):
Cata classic

• Addon 0.20.2

Additional context

Why would you have ElvUI enabled when you want to use Dragonflight?

need uf, namepaltes from elvui
all 'windows' from dragonflight ui

just want to know what generate this error

Ok, that makes more sense.
The error mentions something with trade skills, so have you tried deactivating the dragonflight profession windows in the UI settings? Dragonflight config -> UI -> unset 'Change Profession Window' (and /reload). That might be it.

If you still get errors, try deactivating the complete UI Module; if the errors are only in there, I could maybe add some custom options to deactivate something so its compatible. I havnt used ElvUI ever, so I would need your profile to test more things.