Update Query With If Exists
SQLServer case when statement doesn't work like that. You need a condition, and then a result. Your query only has a condition.

You could rewrite your query like that :

Update #master12
Set [cg] = [dasboot] 
from #master12 m
join [#check1] c1 on m.[uid] = c1.[uid]
WHERE m.[cg] IS NULL

Update #master12
Set [cg] = [redbluegreen] 
from #master12 m
join [#check2] c2 on m.[uid] = c2.[uid]
WHERE m.[cg] IS NULL


Or maybe in a single one like :

Update #master12
Set [cg] = isnull([dasboot], [redbluegreen])
from #master12 m
left join [#check1] c1 on m.[uid] = c1.[uid]
left join [#check2] c2 on m.[uid] = c2.[uid]
WHERE m.[cg] IS NULL


But it will also depend on what you want when you have a uid value present in both tables #check1 and #check2 and which value you want to be used for the update.