After applying the 8.2.0 update, you may observe that some products cannot be added to the shopping cart.

This behavior is observed in products with a recurring price of 0.00.

Please apply the 8.2.1 Maintenance Release via the Utilities > Update WHMCS page.

Adjust the product's recurring price to 0.01 or change the Payment Type option from Recurring to Free.

A hotfix has been created for 8.2.0 only to correct this behaviour:

https://whmcs.community/files/file/165-core-16844-add-zero-price-recurring-products-to-cart/