If you have any employments that don't have the correct age band, follow the below steps to correct this:
Click Configuration then click Settings.
Click Module Settings then click Payroll Settings.
Click National Minimum Wage and drop down the years band.
Click Add Band Age and fill it in for 23+, check a previous year for this to match up and put in this year's wage.
Click Save.
📌Note: You may not want a 23+ band. If you just want 21+ you just need to change the end age on the 21+ band to be 99.