No applicable method for 'st_write' applied to an object of class "c('tbl_df', 'tbl', 'data.frame')" - r

Question

I am trying to transfer data from the Thingspeak API into a postgres database. The API limits each request to 8000 observations, but I need to pull millions! I'm using R to iteratively pull from the API, do a bunch of wrangling, and then submit the results as a data.frame to my table within the db.
The current way I am doing this relies on the dbWriteTable() function from the RPostgres package. However, this method does not account for existing observations in the db. I have to manually DELETE FROM table_name before running the script or I'll end up writing duplicate observations each time I try to update the db. I'm still wasting time re-writing observations that I deleted, and the script takes ~2 days to complete because of this.
I would prefer a script that incorporates the functionality of postgres-9.5' ON CONLFICT DO NOTHING clause, so I don't have to waste time re-uploading observations that are already within the db. I've found the st_write() and st-read() functions from the sf packages to be useful for running SQL queries directly from R, but have hit a roadblock. Currently, I'm stuck trying to upload the 8000 observations within each df from R to my db. I am getting the following error:
Connecting to database:
# db, host, port, pw, and user are all objects in my R environment
con <- dbConnect(drv = RPostgres::Postgres()
,dbname = db
,host = host
,port = port
,password = pw
,user = user)
Current approach using RPostgres:
dbWriteTable(con
,"table_name"
,df
,append = TRUE
,row.names = FALSE)
New approach using sf:
st_write(conn = conn
,obj = df
,table = 'table_name'
,query = "INSERT INTO table_name ON CONFLICT DO NOTHING;"
,drop_table = FALSE
,try_drop = FALSE
,debug = TRUE)
Error message:
Error in UseMethod("st_write") :
no applicable method for 'st_write' applied to an object of class "c('tbl_df', 'tbl', 'data.frame')"
Edit:
Converting to strictly a dataframe, i.e. df <- as.data.frame(df) or attributes(df)$class <- "data.frame", resulted in a similar error message, only without the tbl_df or tbl classes.
Most recent approach with sf:
I'm making some progress with using st_write() by changing to the following:
# convert geom from WKT to feature class
df$geom <- st_as_sfc(df$geom)
# convert from data.frame to sf class
df <- st_as_sf(df)
# write sf object to db
st_write(dsn = con # changed from drv to dsn argument
,geom_name = "geom"
,table = "table_name"
,query = "INSERT INTO table_name ON CONFLICT DO NOTHING;"
,drop_table = FALSE
,try_drop = FALSE
,debug = TRUE
)
New Error:
Error in result_create(conn#ptr, statement) :
Failed to fetch row: ERROR: type "geometry" does not exist at character 345
I'm pretty sure that this is because I have not yet installed the PostGIS extension within my PostgreSQL database. If anyone could confirm I'd appreciate it! Installing PostGIS a pretty lengthy process, so I won't be able to provide an update for a few days. I'm hoping I've solved the problem with the st_write() function though!