ymd_hms / strptime input string too long

I'm trying to convert a column from my dataframe into a Date format. The error I'm getting using ymd_hms() and strptime() both is:

timestamp_df$created <- ymd_hms(timestamp_df$created) Error in strptime(.enclose(x), .enclose(fmt), tz) : input string is too long

The column class is character type and the dataframe size is 13474 rows and 2 columns only.