While helping with a white paper, I noticed that the AASTeX BiBTeX macros were failing (e.g. \aj). The White Paper template is based on the article style, not the aastex style, and @ljones suggested to keep it based on article to keep the page limits consistent.
For others having similar problems (and specifically working in overleaf), you can add the aas_macros.sty file (upload->From external URL->this ADS link) and then add a \usepackage{aas_macros} to your header.
