In RStudio, I cannot get the map created by the following tmap script to save to my working directory. When running the script prior to the tmap_save line, I receive no errors. However, when I run tmap_save, I receive the following error:

Error in tmap_save(st_map, filename = mapname) :
Unknown format. tm should be either a tmap output, or a list of tmap outputs

There are six values for the looping variable (TimePeriod), so I am expecting 6 layers in the map.

Here is my script:

# Map Title
map_title <- paste(agency,'Place Schedule Adherence Map:',bookref,schedule, sep=' ')

# Initialize Map
tmap_mode("plot")
st_map <- NULL

# Loop Through Each Time Period
for (tp in tp_list) {

st_map <= st_map %>%

  tm_shape(st_places %>% filter(TimePeriod == tp)) +
    tm_symbols(col='AvgePgrDelay',
               size=.01,
               scale=10,
               palette = 'YlOrRd',
               group = paste(tp, 'Place Schedule Adherence'),
               interactive=TRUE,
               popup.vars=c('Booking',
                            'ScheduleType',
                            'TimePeriod',
                            'Place',
                            'StopID',
                            'StopName',
                            'Routes',
                            'Observations',
                            'PctEARLY',
                            'AvgeMinutesEARLY',
                            'PctONTIME',
                            'AvgeMinutesONTIME',
                            'PctLATE',
                            'AvgeMinutesLATE',
                            'AvgePgrDelay'))  
 } 

# Add Map Title and Scale Bar
st_map <- st_map %>%
  tm_layout(title = map_title,
            title.size = 10,
            title.fontface = 'bold') 

# Create Map html File
mapname = paste0(agency,
                 ' - Place Schedule Adherence Map - ',
                 bookref, ' - ',
                 schedule,  
                 '.html', sep = '')

# Create Map html File
tmap_save(st_map, filename = mapname)

Thanks for any assistance that you may be able to provide!

I tried running it without the tm_layout command, but still got the same error message.