When adding a new starter or an additional employment for an existing employee you may get the 'Failed to validate' error or the 'Failed to load, check console' error if there is no National Minimum Wage created for the year of the employment.
To resolve this:
Go to Setup and click Settings.
Click Module Settings, then Payroll Settings.
Click National Minimum Wage.
Check to see if there is a band created for the year in which the employment starts. If not, click Add New Year Without Age Bands.
Enter the effective date in the Effective Date field.
Give the NMW band a name in the Description field.
Click the arrow icon for the band you created.
Click Add Age Band.
Add a name in the Description field.
For example, if you are adding the NMW for employees between the ages of 18 and 20, you can name this '18 - 20'.
Enter a start and end age.
For example, 18 as the start age and 20 as the end age.
Enter the value in the Value field.
Click Add Age Band and set up the values for each age range as needed.
Once you've set this up you can go back to add the employment and you will be able to proceed past the Payment screen.