Count rows in a dataframe object with criteria in R - r

Question

Okay, I have a bit of a noob question, so please excuse me. I have a data frame object as follows:
| order_id| department_id|department | n|
|--------:|-------------:|:-------------|--:|
| 1| 4|produce | 4|
| 1| 15|canned goods | 1|
| 1| 16|dairy eggs | 3|
| 36| 4|produce | 3|
| 36| 7|beverages | 1|
| 36| 16|dairy eggs | 3|
| 36| 20|deli | 1|
| 38| 1|frozen | 1|
| 38| 4|produce | 6|
| 38| 13|pantry | 1|
| 38| 19|snacks | 1|
| 96| 1|frozen | 2|
| 96| 4|produce | 4|
| 96| 20|deli | 1|
This is the code I've used to arrive at this object:
temp5 <- opt %>%
left_join(products,by="product_id")%>%
left_join(departments,by="department_id") %>%
group_by(order_id,department_id,department) %>%
tally() %>%
group_by(department_id)
kable(head(temp5,14))
As you can see, the object contains, departments present in each Order_id. Now, what I want to do is, I want to count the number of departments for each order_id
i tried using the summarise() method in the dplyr package, but it throws the following error:
Error in summarise_impl(.data, dots) :
Evaluation error: no applicable method for 'groups' applied to an object of class "factor".
It seems so simple, but cant fig out how to do it. Any help will be appreciated.
Edit: This is the code that I tried to run, post which I read about the count() function in the plyr package, i tried to use that as well, but that is of no use as it needs a data frame as input, whereas I only want to count the no of occurrences in the data frame
temp5 <- opt %>%
+ left_join(products,by="product_id")%>%
+ left_join(departments,by="department_id") %>%
+ group_by(order_id,department_id,department) %>%
+ tally() %>%
+ group_by(department_id) %>%
+ summarise(count(department))
In the output, I need to know the average no. of departments ordered from in each order id, so i need something like this:
Order_id | no. of departments
1 3
36 4
38 4
96 3
And then I should be able to plot using ggplot, no. of orders vs no. of departments in each order. Hope this is clear