I have entered credit memos, however the memos does not get adjsuted with the invoices. In fact I am unable to enter invoice details

I've got your back on managing credit memos in QuickBooks Online (QBO), @vikashvks.

Credit memos are used to reduce the customer's current balance. And it's not like an estimate that can be converted to invoices. Thus, you'll need to apply it to an invoice to pay. 

Since you haven't entered invoice details yet, you'll want to create one. This way, you can apply the created credit memo to the invoice. Here's how to apply the credit memo:

1. Click the + New button, then select Receive payment.
2. Select the customer from the drop-down.
3. From the Outstanding Transactions section, choose the open invoice you want to apply the credit memo to.
4. In the Credit section, select the credit memos you want to apply.
5. Click Save and close.

I'm adding this article for more details: Handle a customer credit or overpayment in QuickBooks Online.

Please let me know if you need clarification about this, or there's anything else I can do for you. I'll be standing by for your response.