To make an invoice, you need to have a client for whom you're invoicing (unless you are doing it just to play around, in which case you can just make up a client).
In your invoice screen you can easily add a new client, without having to switch through too many buttons.
PS: Do you already have an excel file with your goods/services list and would like to upload it directly? Go here to check out how to do this Import my list
Go to the Client name field and click where it says “Select Client”.
Click on +Add New Client (or select one, if you've already added your clients and skip to the next article)
You'll see something like this:
Enter all the relevant details of your client (GSTIN including , if they have it)
Congrats, you just added a new client! You can now invoice this client and you will later on find them in your client list, meaning can create an invoice for them much faster.