Code block causing an error inside the for loop but works outside (R)

Question

for(i in length(refunded_customers$buyer_email)) {
  x <- i+1
  if(refunded_customers$buyer_type[i] == "regular" &&
     any(refunded_customers$buyer_email[i] ==
         refunded_customers$buyer_email[
           x:tail(which(
             refunded_customers$transaction_date == 
               (refunded_customers$transaction_date[i] + 90)
           ), n=1)
         ]
       )
    ) {
      refunded_customers[i,10] <- "not churn"
    }

---

when I use above code I get "refunded_customers$transaction_date[i] + : argument of length 0" error

However when I run the part causing the error outside of the loop (like the one below) it works. Can you identify what the problem is?

---

any(refunded_customers$buyer_email[12]== refunded_customers$buyer_email[32:(tail(which(refunded_customers$transaction_date==(refunded_customers$transaction_date[12]+30)), n=1))])