I was getting issues trying to search for things and tracked down the issue.
If the mod has a github repo or anything I'd be happy to open a PR for the author to look at, but for now I'll just post my fix here.

The localisation is failing inside My Games\Sid Meier's Civilization VI (Epic)\Mods\Map Search Extension\MapSearchPanelExtended.lua

Adding this line:

["GREATWORKOBJECT_HERO"] = "LOC_GREATWORKOBJECT_HERO",

To GreatWorkTypeNameMap (around line 66) fixes the issue.


To prevent further issues popping up in the future I'd recommend modifying AddLocalizedSearchTerm on line 1306 from:
local AddLocalizedSearchTerm = function(kLocKey)
-- TODO try caching localized strings
table.insert( pSearchTerms, Locale.Lookup(kLocKey) );
end
To:
local AddLocalizedSearchTerm = function(kLocKey)
-- TODO try caching localized strings
if (kLocKey == nil) then
print('kLocKey was nil')
print(debug.traceback())
return
end
table.insert( pSearchTerms, Locale.Lookup(kLocKey) );
end


Additionally if you want to be able to search for great works by type (for example: Sculpture) then you can change the loop on line 453 from:
for kGreatWorkType,kGreatWorkList in pairs(kGreatWorkTable) do
AddLocalizedSearchTerm( GreatWorkTypeNameMap[kGreatWorkType] );
for _,szWorkName in pairs(kGreatWorkList) do
AddLocalizedSearchTerm( szWorkName );
end
end
to:
for kGreatWorkType,kGreatWorkList in pairs(kGreatWorkTable) do
typeNameMap = GreatWorkTypeNameMap[kGreatWorkType]
if (typeNameMap == nil) then
print("Unknown kGreatWorkType:" .. kGreatWorkType)
else
AddLocalizedSearchTerm( typeNameMap );
for _,szWorkName in pairs(kGreatWorkList) do
AddLocalizedSearchTerm( szWorkName );
end
end
end

I did this then modified MapSearchPanel.xml and MseQuickResources.lua to list great works as extra resources to select from too which was neat but too much to cover here :p