When you can't pay an employee because 'total taxable annual salary' (Jaarloon BT in Dutch) is missing, you can calculate and fill it in yourself.
Go to their profile and fill it out under Pay > Total Taxable annual salary.
Fill out the total estimated gross salary the employee will earn in the whole year. Add vacation allowance they will earn (8% on top of their gross salary) to it as well. This is an estimation, the Belastingdienst doesn't need the exact number, as you can't know that yet.
This is especially relevant for on-call workers and students, whose annual salary can be more challenging to estimate.
Don't forget to put the 'effective from' date on the first of the month of which you want to pay your employees. So if you are blocked from paying them in June, put the effective from date on June 1st.