Invalid entry

What causes this issue?

You want to edit a document and have entered the special character
"<LB>*</> in the field <LS>Document</>. This entry is not allowed.

How to fix this error?

Correct your entry in the field <LS>Document</>. Note the following
information on special characters:
All special characters are supported for the document number.
The only exceptions are: "<LB>*</>", "<LB>,</>" and "<LB>&</>".
The special character "<LB>*</>" has a particular meaning in the
<LS>Create</> function:
If internal number assignment has been defined in Customizing for the
document type concerned, you can enter this special character at the
beginning of the field. The document number will then be assigned
internally by the system.

Error message extract from SAP system. Copyright SAP SE.