When applying the 2020 release wave 1 you may see a component such as the Dynamics 365 Core Service fail to complete.
First, you may want to check that you have correctly followed the steps on how to opt-in for 2020 wave 1.
To determine the issue - navigate to the solution manager in PowerApps and click 'See History'
This should then show you the failed upgrade component:
Clicking on the row will give you the details. In my case it was because the solution was blocked due to a previous upgrade being incomplete:
Solution manifest import: FAILURE: The solution [FieldService] was used in a LayerDesiredOrder clause,
but it has a pending upgrade.
Complete the upgrade and try the operation again.
To resolve this, you will need to navigate to the solution manager and click 'Switch Classic'. Locate the referenced solution that is still pending an upgrade, select it, and then click 'Apply Solution Upgrade'.
Wait for the upgrade to be applied, then return to the 2020 wave 1 release area in the admin portal, and click 'Retry'
If you see a subsequent error, you can repeat these steps for the failed solution.
Hope this helps!