Hi,
If you are manually inputting sales, you can do this successfully, but you must input all of the required fields (or it will lead to this issue you are having).
The required fields that need to be filled in for manual entries are:
(Note: you can fill in as much additional information as is suited for the entry, but these need to be filled in at minimum)
"Item/Transaction ID (Unique)" - this can be filled in with any unique value
"Quantity" - self exlanatory
"Sale Date" - self explanatory
"Month Sold" - First day of the month of Sale date, can be populated by formula if you choose
"Sale Price" - self explanatory