Solution
For the sow in question, it is possible to enter an 'Out of production' date, but a 'Removal date' cannot be entered. The sow is still lactating and must be weaned first.
If the 'Out of production' date has already been entered, it must be removed first:
Go to '1. input/Edit, 1.1. Sows, 1.1.1. Arrival/Removal'
Click 'Search' and select the animal.
Clear the date in the 'Out of production date' field.
Next, the sow must be weaned:
Go to '1. input/Edit, 1.1. Sows, 1.1.5 Enter mating/farrowing/weaning data (individual)'
Click 'Search' (icon: pig with magnifying glass) and select the animal.
Enter the weaning data on the 'F6 - Farrowing/Weaning' tab.
Go to: '1. input/Edit, 1.1. Sows, 1.1.1. Arrival/Removal'
Click 'Search' and select the animal.
Enter a date in the 'Removal date' field.