I want to filter the second pick list according to the first one. For example, if I choose department = Human resources, I want the second picklist to show only the employees that belong to that department.
In my table there is already a filter linked to the deparment
Hi @Regi-d60a, welcome the community and thanks for your post! 
I'm happy to show you how to set this up 
First, create a table of Departments and another of Employees, which has a column that is a picklist of the Departments table:
Then in the app, add a FILTER formula to the second picklist variable to only show employees of the selected department. This is the formula for my example: =FILTER(Employees, "Employees[Department]=$[DepartmentSelector]")
This is how it looks in the app:
Hope that helps! Let us know if you have any further questions. 