99: Undeclared custom mark

An ampersand & is used for custom marks in the text. A custom mark consists of the ampersand immediately followed by a small or capital letter, az or AZ, or a single digit 09. The custom mark must be declared in a metadata declaration. In your case, it is not yet declared.

Solution

Make sure that you declare the custom mark. Note that its name is case-sensitive, so &a and &A are different marks. The following example demonstrates how &1 is declared to represent a HTML word break and how it is then used twice in a long word:

@ &1: ->,PHdicj4=

The patient suffers from sesqui&1pedalo&1phobia.

If you do not want to use a custom mark, but want a regular ampersand instead, you can prepend a backslash: \&