Solution
Go to:
[1. input/Edit] - [1.1. Sows] - [1.1.5. Mating/farrowing/weaning data (Individual)]
Press 'F3'
Enter the sow's plastic tag number. Select the correct cycle.
Click the 'F9 - Individual piglets' tab.
Click the 'Tattoo individual piglets' button to tattoo the piglets.
Check the last used number.
Enter the number of piglets to be tattooed.
Click the green checkmark to confirm.