You may see this or a similar error for elements such as <aff> or other linked objects whose ID value is created using the Article ID of the manuscript. This warning tells you that the ID attribute for the affiliations (or whatever linked object) doesn't start with a letter. All IDs must start with a letter, per the NLM DTDs (in fact, it's an XML rule that identifiers may not start with a digit).
You are more likely to see this message if the the ID attribute for affiliations (or other linked objects) is concatenated from the article ID, and the article ID is the first segment of the ID value.
For example, in a configuration whose affiliations ID attributes is concatenated using this formula:
article ID + aff + sequence number
then the article ID must begin with a letter.
To resolve this issue, please change the Article ID of the manuscript to a value that begins with a letter.