The message 'No position at effective date' signals that the employee does not have an active employment on the effective date of the salary you are adding.
For example, their post may have an end date of 31st August while the salary is being added for the 1st September.
To check this:
Search for the employee and go to their profile.
If they have multiple employments, click Switch Employment and select the one you need to add a salary for.
Click Employment and then Position.
Select Position History.
Check the end date of the employee's current post.
To resolve this:
On the employee's profile, click on Admin and select Admin.
Click Edit an existing post/position.
Select the relevant post.
Remove the date in the End Date field, or amend it as necessary.
Click Save.
Once the above steps are completed you will be able to change the employee's basic pay as needed.