I have a temporal table
var data = Conn.ExecuteReader("select * from Employee e FOR SYSTEM_TIME ALL WHERE e.Id=15");
Incorrect syntax near FOR
Problem is you are using table alias
e and so the error. Don't think you can use table alias. change it to
select * from Employee FOR SYSTEM_TIME ALL WHERE Id=15
If you check the documentation Querying Data in a System-Versioned Temporal Table (OR) Temporal Tables you will see that the syntax doesn't show the use of table alias at all. Rather you will have to use the entire table name.
See this related post as well Why does the FOR Clause not work with an alias in SQL Server 2016 with temporal tables?