Numeric value is not recognized

Trying to execute a query like this and getting the numeric value not recognized error. The datatypes for the columns used are

Col1 - VARCHAR, 
Col2 - VARCHAR, 
Col3 - TIMESTAMP_NTZ(9)

This is my query

Select count( distinct ( 
                     case when Col1 > 100 
                     then concat(col2,cast(col3 as string))
                     else 0
end))
                    

your CASE is returning a string and a number which are not the same types, so if you make the 0 into a string '0' that should work better.

A side note, you will still have a count of 1 for the all the values <= 100, which seems somewhat strange. Where-as if you used null instead of the 0 then it will not count, and will not have a type problem.