I am trying to bill, but from the details section adding the purchase order, not working

Let's make sure that you're able to link the PO when creating the bill, Ram_28.

There are times that browsing issue would hide buttons or prevent QBO users from performing a task. This might be the reason why you're unable to add the Purchase Order when creating the bill. 

If you have a different browser, use it to create the transaction. You can also switch to an incognito or private browsing session while downloading transactions. Use these keyboard shortcuts to start an incognito or private browsing session: 

• Ctrl + Shift + N  for Google Chrome
• Ctrl + Shift + P for Firefox
• Control + Option +P if you're using Safari
• Ctrl + Shift + P for Microsoft Edge

After that, go back to your account check if you can add the PO. If you're able to do it, go back to the regular browsing session and delete your temporary internet files. Here's an article as a guide: Clear Cache And Cookies To Fix Issues When Using QuickBooks Online.

Additional references just in case you need to review your vendor-related transactions are also shared here: 

• View Supplier Transactions 
• Run a Report with Supplier Totals 

Let me know if you have more questions to ask. We're always here to guide and help you again.