It seems geom_smooth will not calculate a line if the x values are a factor and Weekday is a factor. It is fairly easy to work around that limitation in this case. Note that I am plotting the data frame dailymerged from which ActivityLong was built. It contains the three values of Weekday and TotalMinutesAsleep in individual columns so they are convenient for plotting. In ActivityLong, the Weekday and TotalMinutesAsleep numbers are repeated many times, which is not an accurate representation of your data.
To get geom_smooth to plot, I used as.numeric(Weekday) to extract the numeric value of the factor. I included an offset of - 2 as an inaccurate hack to make the plot look approximately correct. You can see why from this code.
as.numeric(dailymerged$Weekday)
[1] 3 4 6
Your three days of Tuesday, Wednesday, and Friday have numeric values of 3,4,6, since Sunday = 1. On the plot those three factor values are placed in the positions 1, 2, and 3. By subtracting two, I get the first two to line up correctly and you can see the general idea of the plot. If your real data have all the days of the week, you will not have to worry about any offset.
library(tidyverse)
#> Warning: package 'tibble' was built under R version 4.1.2
dailymerged <- structure(list(Id = c(1503960366, 1503960366, 1503960366),
Date = structure(c(16903, 16904, 16906), class = "Date"),
Weekday = structure(c(3L, 4L,6L), .Label = c("Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"), class = c("ordered", "factor")),
Calories = c(1985L, 1797L, 1745L),
TotalSteps = c(13162L, 10735L, 9762L),
TotalDistance = c(8.5, 6.96999979019165, 6.28000020980835),
VeryActiveDistance = c(1.87999999523163, 1.57000005245209, 2.14000010490417),
ModeratelyActiveDistance = c(0.550000011920929, 0.689999997615814, 1.25999999046326),
LightActiveDistance = c(6.05999994277954, 4.71000003814697, 2.82999992370605),
SedentaryActiveDistance = c(0, 0, 0),
VeryActiveMinutes = c(25L, 21L, 29L),
FairlyActiveMinutes = c(13L, 19L, 34L),
LightlyActiveMinutes = c(328L, 217L, 209L),
SedentaryMinutes = c(728L, 776L, 726L),
TotalSleepRecords = c(1L, 2L, 1L),
TotalMinutesAsleep = c(327L, 384L, 412L),
TotalTimeInBed = c(346L, 407L, 442L)),
row.names = c(NA, 3L), class = "data.frame")
ggplot(dailymerged, aes(x = Weekday, y = TotalMinutesAsleep,)) +
geom_point()+
geom_smooth(formula = y ~ x, method = "lm",aes( x = as.numeric(Weekday)-2))

Created on 2022-05-23 by the reprex package (v2.0.1)