Problem in Date/Time while calculating daily average from hourly average using group_by in R

I have calculated hourly average using the following code in R. Hourly average is stored in an object 'avghr'. Because of the data preprocessing done to obtain 'merged' followed by 'avghr', I want to use 'avghr' to calculate daily average. I am facing error 'Error in group_by():
ℹ In argument: Time_sp = lubridate::floor_date(Time_sp, "1 day").
Caused by error in lubridate::floor_date():
! object 'Time_sp' not found'.
Could anyone please help me in resolving the error.

Code:

 merged <- rbindlist(dt.tidied, fill = TRUE, use.names = TRUE)
 
 #Required Columns
 cn <- c('Date/Time', 'C1 (C1)', 'C2 (C2)', 'C5 (C5)', 'C8 (C8)', 'C11 (C11)', 'C14 (C14)')                                      

 #Calc 15min average
 avghr <- merged %>%
 select(any_of(cn)) %>%
 as_tibble() %>%
 group_by(Time_sp = lubridate::floor_date(`Date/Time`, "60 mins")) %>%               #I want Date/Time column to be written as Time_sp in the output.
 #mutate(Time_sp = format(Date_Time, "%Y-%m-%d %H:%M:%S+00")) %>%                    # #I want Time_sp column to be written in the specified way. 
 summarise(across(where(is.numeric), ~ if(mean(is.na(.x)) > 0.5) NA else mean(.x, na.rm = 
                                                                               TRUE)))

write.csv(avghr, paste0(dirlist[idx],"_hr.csv"), row.names = FALSE)


 #Calc daily average
 avgdl <- avghr %>%
 select(any_of(cn)) %>%
 as_tibble() %>%
 group_by(Time_sp = lubridate::floor_date(`Time_sp`, "1 day")) %>%
 #mutate(Time_sp = format(Date_Time, "%Y-%m-%d %H:%M:%S+00")) %>%
 summarise(across(where(is.numeric), ~ if(sum(is.na(.x)) > 1) NA else mean(.x, na.rm = 
                                                                               TRUE)))

write.csv(avgdl, paste0(dirlist[idx],"_dly.csv"), row.names = FALSE)

}

I understand that the mutated Time_sp column is no longer in Date/Time format. But even if I remove the particular code line mutate(Time_sp...) while calculating avghr, it still gives me the same error.

I am able to calculate hourly or daily average individually using the same code. The problem is occurring while using hourly average object to calculate daily average. Can anyone please help.

Output of dput(head(avghr))

structure(list(Time_sp = structure(c(788918400, 788922000, 788925600, 
788929200, 788932800, 788936400), tzone = "UTC", class = c("POSIXct", 
"POSIXt")), `C1 (C1)` = c(84.6666666666667, 90.95, 94.7333333333333, 
95.95, 95.4666666666667, 90.3833333333333), `C2 (C2)` = c(NA_real_, 
NA_real_, NA_real_, NA_real_, NA_real_, NA_real_), `C5 (C5)` = 
c(NA_real_, NA_real_, NA_real_, NA_real_, NA_real_, NA_real_), `C8 
(C8)` = c(911.983333333333, 1062.41666666667, 1147.88333333333, 
1156.15, 1089.73333333333, 956.233333333333), `C11 (C11)` = 
c(NA_real_, NA_real_, NA_real_, NA_real_, NA_real_, NA_real_), `C14 
(C14)` = c(1139.86233333333, 1304.17933333333, 1394.967, 
1406.02683333333, 1336.59633333333, 1191.40616666667)), row.names = 
c(NA, -6L), class = c("tbl_df", "tbl", "data.frame"))

#Calc 15min average
avghr <- merged %>%
  select(any_of(cn)) %>%
  as_tibble() %>%
  group_by(Time_sp = lubridate::floor_date(`Date/Time`, "60 mins")) %>% 
  summarise(across(where(is.numeric), ~ if(mean(is.na(.x)) > 0.5) NA else mean(.x, na.rm = TRUE)))

mutate(avghr, Time_sp = format(Time_sp, "%Y-%m-%d %H:%M:%S+00")) %>%
  write.csv(paste0(dirlist[idx],"_hr.csv"), row.names = FALSE)

#Calc daily average
avgdl <- avghr %>%
  group_by(Time_sp = lubridate::floor_date(`Time_sp`, "1 day")) %>%
  summarise(across(where(is.numeric), ~ if(sum(is.na(.x)) > 1) NA else mean(.x, na.rm = TRUE))) %>% 
  mutate(Time_sp = format(Time_sp, "%Y-%m-%d %H:%M:%S+00"))

write.csv(avgdl, paste0(dirlist[idx],"_dly.csv"), row.names = FALSE)