Is there a time limit to cancel an e-bill after generating it?How do I generate invoices in BUSY?

Yes, there is a time limit to cancel an e-invoice after generating it. The time limit is within 24 hours of generation. 
To generate E-invoices in BUSY, you need to follow the below provided steps:
Step 1: You have to create a GSP (GST Suvidha Provider), you need to visit the eway bill portal. 
Step 2: Click on regisration then click on gsp then create id with Selecting Webtel Electrosoft Private Limited and Pinnacle Finserv Advisors Private Limited as the GSP Vendor type.
Step 3: Go to Administration and click on configuration.
Step 4: Select Features / Options and click on GST / VAT, here you will see a option called E-invoice Required.
Step 5: Set Y to turn it on and mention your place and pincode,user restrictions you can define and then clck on GSp configuration Tab then paste GSP username and password and save the configuration
Step 6: Go back to Administration and click on configuration.
Step 7: Choose the Voucher Series Configuration option and select the voucher type for which you want to generate e invoice, such as sales, sales return, etc. and select series & then click on voucher configuration then enable generate E-invoice after saving and save.
Step 8: Open or add your sale invoice then you have to enable E-invoice required and fill all details of a voucher such as pincode station on transporter window
Step 9: after saviing voucher one window will appear for generate e-invoice 
Step 10: check all possible errors and then generate e-invoice.