To make an error characteristic of beginners and showing a lack of experience. Failing to control the center squares in chess in the first move or two is often a “rookie mistake”. Not reading the details of an online shipment and getting a subpar item when first online shopping is a “rookie mistake”, etc.
A 'rookie' refers to someone who is new at something, someone who does not have the experience or level of training some of the older workers may have. To give some context, the opposite of rookie is professional. Rookies are more likely to make mistakes professionals would not because professionals have more knowledge or experience.
The term 'rookie mistake' is used to refer to any mistake made that is so easily prevented that only someone with as little experience as a rookie would make it. It can be used as an insult between professionals who want to show that they know better or are better at doing something than another professional.