You select dept , but the DBMS want to know whether you mean employees.dept or department.dept , despite them having the same value in your query.

It is good practise to qualify all columns with their table name when working with multiple tables:

select employees.first_Name, employees.salary, employees.dept
from employees
inner join department on employees.dept = department.dept;

(Which shows that the join to the department table is completely superfluous as yet.)

As the long table names can make queries harder to read, it is also good practice to use mnemonic table aliases:

select e.first_Name, e.salary, e.dept
from employees e
inner join department d on e.dept = d.dept;