I am unsure which column of your data you want to use to color the lines. If it is cpid, then add color = cpid to the aes()
Fsigt6a %>% filter(is.na(flag)) %>%
ggplot(aes(12*dptp1/365.25, AIRg, group=cpid, color = cpid)) +
geom_line() +
geom_smooth(group = 1, se = F, color = 'red') +
labs(x = "Months relative to first PHPI transplant",
title = "Course of AIRg, CIT-06 subjects")
If that is not what you want, please make a Reproducible Example of data and explain which column designates a group and which a class.