Why am I unable to generate the e way when the buyer and seller are in different states?

If you are unable to generate the e-invoice when the buyer and seller are in different states it could be several reasons:
 Step 1:If your party and you are from different states, IGST will be applicable to your transaction, select sale type as IGST Then generate, it will be generated.
 But In some cases, if you want to charge local tax instead of central tax, you will need to enable billing and shipping in your BUSY.
 Step 2: Go to Administration menu,
 Step 3: Select Configuration,
 Step 4: Select voucher series configuration,
 Step 5: Click on sales select series and open voucher configuration tab,here is a option enable billing shipping details then specify Y then save. 
 Step 6: After that while generating e invoice you will see there after selecting a party,Billing shipping window will appear, If your party is from another state but you are shipping the goods local, and you want to apply local GST, you need to select GST report basis as per billing shipping in the billing and shipping window. You should also mention the local address in it, then click generate automatic here you will see the option is pick place of supply from billing shipping Details specify Y Then proceed to generate it will be generated successfully.