Hi Giang,
You can only record quantities in a Customer BP Invoice line when the analytical account manages "non financial unit entry".
Example with Seed data.

Account NA 41100

Hello Vincent Labat Can the 'Tax' be inputted/edited in the Customer BP invoice? I am not sure about this process

See it here https://communityhub.sage.com/us/sage_x3/f/general-discussion/219526/customer-bp-invoice-function-field-tax-allows-for-display-input