Only use the characters 'A..Z' '0..9' and '_'; Elements -> Long Text

What causes this issue?

You have used characters that are not permitted for a key field.
For technical reasons, you may only use the following characters for
key fields 'A..Z', '0..9', and '_'. You must not use blanks or any
other characters.
Element labels must comply with the XML 1.0 naming conventions because
they are saved as XML tags in the expert profile.
The following restrictions apply:
The first character must be a letter.
The label may not begin with 'XML'.

System Response

You cannot save the entry.

How to fix this error?

Choose a label that fulfills the requirements described above.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.