Not all variables bound ORA-01008: not all variables bound error
Maybe you have set define to off.

set define off
select last_name, salary from employees where employees.salary&gt;&amp;sal;

Error starting at line : 2 in command -
select last_name, salary from employees where employees.salary&gt;&amp;sal
Error at Command Line : 2 Column : 31
Error report -
SQL Error: ORA-01008: not all variables bound
01008. 00000 -  "not all variables bound"
*Cause:    
*Action:


Set it to on (which is the default value by the way):

set define on
select last_name, salary from employees where employees.salary&gt;&amp;sal;


With define set to on, SQL Developer will prompt for a value for the variable &amp;sal.