Works OK for me:
SQL> create table "Employee_Master" ("Start Date" date);
Table created.
SQL> insert into "Employee_Master" values (sysdate);
1 row created.
SQL> declare
2 the_variable date;
3 begin
4 select min("Start Date") into the_variable
5 from "Employee_Master";
6 end;
7 /
PL/SQL procedure successfully completed.
SQL>
Though, why, oh, why are you making your life miserable? Avoid double quotes while working with Oracle. If you do that, you always have to reference tables (and columns) using double quotes and matching letter case exactly as during creation process.
By default, Oracle stores everything as uppercase, but you can reference those objects any way you want (upper, lower, mixed case - doesn't matter). But, with double quotes, as I've said - exact matching is required.
By the way, screenshot you attached shows a lot of nothing and just a little bit of something. Couldn't you have taken a better screenshot?
begin null; end;- Sayan Malakshinov/in the next line - Sayan Malakshinovdeclare dt date; begin dt:=date'2020-01-01'; end;with/in next line - Sayan Malakshinov