Bailey attends her 20th high school reunion and realizes she'd have great difficulty with names if people weren't wearing nametags. Her inability to remember the names of people she spent so much time with 20 years earlier is MOST likely due to the sin of:

Sin of transience

Explanation:

Transience is the decrease in the ability of the memory to access information over time. In other words, when the memory deteriorates I've time. Transience is a kind of sin of omission.

In transience, the memory is usually affected by decay and aging. An individual who was able to remember past information, events or locations is only capable of remembering present information than the past information.

In her case, she wasn't able to remember the names of her classmates after about 20 years as a result of sin of transience. This could be as a result of aging or the fact that she couldn't just retrieve those information because it has been a long time.