In the part 1.PeriodAccumulation, the path of the experimental dataset was wrong. It was empty and the function CST_Load() wasn't working in the first code block, as it was mentioned by @aho in Issue #15. I change it from:
S5path_prlr <- list(path = '/esarchive/exp/ecmwf/system5c3s/original_files/chou/daily_mean/$VAR_NAME$_s0-24h/$VAR_NAME$_$YEAR$$MONTH$01.nc')
into:
S5path_prlr <- list(path = '/esarchive/exp/ecmwf/system5c3s/$STORE_FREQ$_mean/$VAR_NAME$_s0-24h/$VAR_NAME$_$START_DATE$.nc')
Alos, I added into the file paths the variable $STORE_FREQ$ that was specified in the arguments but it was not used in the path.