Upgrading from 16.2 to 17.3

Attempting to upgrade Zorin 16.2 to 17.3 but get "Sorry, an internal error happened. The upgrader couldn't complete checking the upgrade requirements". The report reads as follows:

[CHECKING] Started
[CHECKING:Validating Upgrade Attempt] Started
[CHECKING:Validating Upgrade Attempt] Completed
[CHECKING:Checking Filesystem] Started
[CHECKING:Checking Filesystem] Completed
[CHECKING:Creating Necessary Files] Started
[CHECKING:Creating Necessary Files] Completed
[CHECKING:Adding GPG Keys] Started
[CHECKING:Adding GPG Keys] Completed
[CHECKING:Restarting Apt] Started
[CHECKING:Restarting Apt] Completed
[CHECKING:Simulating Flatpak Transaction] Started
[CHECKING:Simulating Flatpak Transaction] Completed
[CHECKING:Processing Apt Repos] Started
[CHECKING:Processing Apt Repos] Completed
[CHECKING:Updating Apt Repos] Started
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Retrying
[CHECKING:Updating Apt Repos] Fatal Error: { error-repo-download-failed: }
[CHECKING:Updating Apt Repos] Failed
[CHECKING] Failed
[CHECKING:Restoring System] Started
[CHECKING:Restoring System] Error: { error-repo-download-failed: }
[CHECKING:Restoring System] Completed

Could anyone help please?

Welcome to the Forum!

Did You added any Third-Party Repos? Can You run in Terminal sudo apt update && sudo apt upgrade and post the Output here please?

Hi. I tried pasting the (very extensive) output, but the post got declined because as a new user, I can only post a maximum of 5 links. Removing the https: from the hits didn't make a difference.

Ah, okay. You have to Options. Copy the Stuff in the Comment, mark all of it and click on this Button:
grafik

That will switch the Style. Alternatively, You could take Your Phone and make a Picture and post that where.