Problems with a Comma in Jira User Names
Question: How to avoid errors when the Jira user names contain commas?
Answer: The errors occur because when Jira user names include commas, e.g. "Smith, John", they are treated as list separators by MS Project, so it thinks that Smith and John are two separate resources.
There are a few ways to work around this:
Map Jira users to MS Project resources using a custom resource field:

and populate the field with Jira user names. This way, the names will be treated as a single string and not split by commas.

Map Jira users to MS Project resources using e-mail or Data Center login to avoid formatting issues.
Adjust the list separator character in your Windows regional settings.