You cannot create an exception for this column group

What causes this issue?

You have tried to create an exception for a column group. This is not
possible with certain currency settings.
If an exception is to be created for a column group, this presupposes
that all relevant columns are uniform with respect to database
currency, list currency and currency translation key.

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?

You have two possibilities:
You can create exceptions to individual columns.
You can convert the currencies of the affected columns uniformly.

Error message extract from SAP system. Copyright SAP SE.