Unable to make outbound calls from Desktop App

sounds like you are using a free extension without a license.
make sure you assign a license to it. Here are 2 ways to add a license to a user extension:
Method1
Admin Portal > Users > click on Actions for a given user (3 vertical dots) > Edit licenses

Method2
Users > click on a user > Devices & numbers > you should see “No phones are available” > click on Add phone

in the pop up for adding a phone one of the steps is to select a phone. Here you should select “Other phones”  and then “RingCentral app”.