ggplot2 geom_smooth

I think this is happening because you are just passing data to geom_point() and not globally although I can't be sure since you are not providing a reproducible example.

Try with this

ggplot(data=fourseamers1, aes(x=spin_rate, y=swstrike)) +
    geom_point(color= "light green") +
    ylim(0,.2) +
    xlab("Average Spin Rate") +
    ylab("Swinging Strike Percentage") +
    ggtitle("4-Seam Fastball")+
    theme(plot.title = element_text(hjust =.5)) +
    geom_smooth(method = 'lm')

If this doesn't work, please provide a minimal REPRoducible EXample (reprex). A reprex makes it much easier for others to understand your issue and figure out how to help.

If you've never heard of a reprex before, you might want to start by reading this FAQ: