I'm having the same problem with the same exact dataset. When I get the error, the Environment also defaults to an older version that is missing my "heartrate" dataframe but includes older dataframes that I'd deleted.
@M_AcostaCH , the dataset is from the "heartrate_seconds_merged" file that is part of the FitBit Fitness Tracker Data hosted here. Below is the code I used to try and separate the Time column.
heartrate <- read.csv(file = "Datasets/heartrate_seconds_merged.csv")
heartrate <- separate(heartrate, Time, into=c('HRDate', 'HRTime'), sep=' ')
I also did the same thing with the "sleepDay_merged" file, and I did not run into the same error, so I don't know why it's wrong. Here's the code for the sleepDay separation for comparison.
sleep <- read.csv(file = "Datasets/sleepDay_merged.csv")
sleep <- separate(sleep, SleepDay, into=c('SleepDate', 'SleepTime'), sep=' ')