A screenshot isn't very useful, try to give us a better dataset following this isntruction:
Second: The dataset comes outside the aes() and you don't have to use the dollar-notation but can call the columns directly, also you need to define a x and y value for geom_point()
p <- ggplot(thesisdate,
aes(x = date , # (???)
y = salinity, color=stream_location) +
geom_point() +
facet_grid(rows = vars(water_system), cols = vars(session),
scales = "free") +
theme_classic())