for the coord_() func c("2020-07-02", "2020-07-09") is not a vector of two dates, rather of two strings.
similarly scale_x_datetime() will be trying to interpret the vector in your data.frame mapped to the x aes-thetic as a POSIXct datetime, if you passed character string with the mere appearance of dates you would expect to get such errors.
base R has some functionality to convert to Date type, i.e. as.Date(), however many prefer the convenience supplied by lubridate package, the ymd() function will easily interpret your character strings as Date's. I think the answer here is to step back from the ggplot code and wrangle your data passed in a little more first.
For more detailed support, I would ask that you provide a reprex.
Hope this helps.