Characteristic & contains characters that are not allowed

What causes this issue?

You have chosen a name for a characteristic that contains characters
that are not allowed.

How to fix this error?

The following naming conventions apply to characteristics:
You can only use letters from A - Z, figures from 0 - 9, and the
underscore character.
You can also use hyphens in characteristic names. However, if you do
so, you can only refer to the characteristic in object dependencies
with the keyword SKEY. You enter the characteristic name in quote
marks.
Example: SKEY 'EXT-COLOR'
The first character of the name must be a letter.
The characteristic name cannot consist of only figures.
You cannot use umlauts.

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