Product to line allocation in R using ompr. Error: The expression contains a variable that is not part of the model

Question

I am working on an allocation problem using R s ompr package. The aim is to allocate Products to production lines. I have written the following code for this:

volume <- product$VolumeQ4          #The volume of quarter 4 of each of the products
capacity <- lines$Capacity          #The capacity of the line in quarter 4
k <- length(product$k)              #The products (k) 54 in total 
l <- length(lines$l)                #The lines (l) 11 in total
product_lines_matrix                #this is a 54x11 dataframe showing if a line is capable 
                                    #to produce a product, 1 = if capable, 0 if not capable. 

model <- MIPModel() %>%
  
# 1 if product i  is assigned to line j 
  
add_variable(x[i, j], i = 1:k, j = 1:l, type = "binary") %>%
  
  # # objective is to assign all skus to lines where the line is capable
  
  set_objective(sum_expr(product_lines_matrix[i, j] * x[i, j], i = 1:k, j = 1:l), "max") %>%
  
  # each product needs to be assigned to line 
  
  add_constraint(sum_expr(x[i, j], j = 1:l) == 1, i = 1:k) %>% 
  
   # we cannot exceed the Q4 capacity of a line
  
  add_constraint(sum_expr(x[i, j]*volume[i], i = 1:k) <= capacity[j], j = 1:l) 

I am getting the following error code

Error in check_for_unknown_vars_impl(model, the_ast) : The expression contains a variable that is not part of the model.

I am not used to modeling MIP in general, let alone in R, but would appreciate any help I can get on this! Any ideas where I go wrong ?