I have enabled multi currency in quickbook but when I create an invoice its showing only home currency PKR and not allowing me to create an invoice in USD

QuickBooks treats each customer as having one "assigned" currency. If you want to bill someone in USD, that customer must be set up as a USD customer. 

To do this, go to All apps > Customer hub > Customers and Leads > New customer, then under the Payment and billing tab, select USD as their currency before saving.

Note that you cannot change the currency for an existing customer once they have been created.

If you have any further questions, feel free to comment below.