Issue with loop: argument of length 0 - r

Question

I've tried working on this loop and come out with the below errors. I'm not sure if I can provide data, if needed I'll do my best to obfuscate the data. Here is the loop I am trying to use, any tips on what I'm doing wrong would be greatly appreciated as I haven't found a viable solution yet. The exact error is below the code.
decay_function = function(df)
{
df <- df[order(df$department,df$product,df$region,df$monthnum),]
for(mk in 1:ncol(levels_department)) {
newdata <- df[which(df$department==as.character(levels_department[,mk])), ]
levels_product<-as.data.frame(t(levels(as.factor(newdata$product))))
for(md in 1:ncol(levels_product)){
newdata <- newdata[which(newdata$product==as.character(levels_product[,md])), ]
levels_region<-as.data.frame(t(levels(as.factor(newdata$region))))
for(dm in 1:ncol(levels_region)){
newdata <- newdata[which(newdata$region==as.character(levels_region[,dm])), ]
for(i in 1:(nrow(newdata)-1)){
start_month = newdata$monthnum[i]
end_month = newdata$monthnum[nrow(newdata)]
row_vector = c()
decay_vector = c()
for(j in 5:ncol(newdata)){
k = 0
for(l in start_month:end_month){
distance_initial = (l - start_month)
vector_increment = (l - (start_month-1))
decay_rate = (0.5)^((1/halflife)*distance_initial)
decay_value = (decay_rate)*(newdata[[i,j]])
k = k + decay_value
}
df2[i,j] = k
}
print(df2)
}
if (mk=='1' & md=='1' & dm=='1'){
outdata<-df2
} else {
outdata<-rbind(outdata,df2)
}
}
}
}
}
output_data = decay_function(tempone)
Error in start_month:end_month : argument of length 0
> dput(head(df))
structure(list(monthnum = c(33, 33, 33, 33, 33, 33), Region = c(2251,
2251, 2251, 2251, 2251, 2251), Department = c("Softlines", "Softlines",
"Softlines", "Softlines", "Softlines", "Softlines"), Product = c("T-Shirt",
"Jacket", "Sweat Shirt", "Tank Top", "Sweat Pants", "Mens Jeans"
), Incentive_Amount = c(5742.43, 108006.61, 459076.67, 34006,
141632.42, 29580.38), Leads_T1 = c(0, 0, 0, 0, 0, 0), DCLeads = c(0,
1, 0, 0, 0, 0), PhoneLeads = c(0, 0, 0, 0, 0, 0), T3_CRM_Leads = c(0,
0, 0, 0, 0, 0), Leads_Third = c(0, 1, 0, 0, 0, 0)), class = c("tbl_df",
"tbl", "data.frame"), row.names = c(NA, -6L))