Bill period can be added for the manually generated invoices as well, so kindly follow the below steps to add the billing period.
Step 1: Go to Income Tracker >> Set up >> Customize Bill.
Step 2:Scroll down at the bottom and against "Bill Period & Frequency" click on 'Create New bill period'.
Step 3:In the new window Enter the From and To bill number, Bill period>>Save changes.
After which the same bill period will be seen in the selected invoices.