Error CS0246 "'IronSourceImpressionData' could not be found."

I’m guessing that’s part of advertising so if you don’t need it, remove it.

Otherwise, make sure all packages are up to date and that they resolved fully, perhaps try a reimport-all.

Here’s more reading:

Extra unwanted packages in new projects (collab, testing, rider and other junk):

Upgrades:

If you need packages and they’re not working, one easy solution is to make a new project, add the packages there until they work, then copy the Packages/manifest.json file to your current project. This obviously assumes you are using source control and can instantly back out any and all changes!!

New project:

About the fastest way I have found to make a project and avoid all this noise is to create the project, then as soon as you see the files appear, FORCE-STOP (hard-kill) Unity (with the Activity Manager or Task Manager), then go hand-edit the Packages/manifest.json file as outlined in the above post, then reopen Unity.

Sometimes the package system gets borked from all this unnecessary churn and requires the package cache to be cleared: