Distribution key & is not a valid reference

What causes this issue?

You can only refer to one of the following standard distribution keys:
0 Manual distribution
1 Equal distribution
2 Analog distribution
3 Percentage distribution
4 Distribution of values to subsequent periods
5 Copying values to subsequent empty periods
6 Carrying forward single values
7 Distribution according to the number of days per period

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.