'&' cannot be interpreted as year

What causes this issue?

The value '&V1&' was entered for the construction year. This value
cannot be interpreted as being a year.

System Response

Your entry was not accepted by the system.

How to fix this error?

Enter the construction year in a four figure form. You have to enter
the day, month and year if the construction year is the same as the
start or end date of a building age category in the respective
representative list of rents.

Error message extract from SAP system. Copyright SAP SE.