Can Balance Sheet Comp show the prior year end, rather than the same date as the current YTD, when I select "prior period, rather than prior year (as desktop does)?

Don't use the "Compare another period" option.  Select the report period from the 1st day of LY to the end day of YTD, and select "Display columns by": Years

I could run this report in QB 2018 but it does not open in 2021. Trying to make a new one I can't get it to show the difference Between YE and YTD $ and %.  Suggestions to fix the issue?

I'm here to ensure you're able to pull up the Balance Sheet Comparison report in QuickBooks Desktop (QBDT), @EGGonTF. This way, you can get the necessary financial information you need.

You may have minor data issue with your company file using the QBDT 2021 version. Don't worry because the QBDT software has built-in tools (Verify and Rebuild) that'll help automatically fix company data matters. To do this, here's how:

1. Go to the File menu.
2. Select Utilities and then choose Rebuild Data.
3. On the QuickBooks Information window, select OK.
4. Follow the onscreen steps to save a backup. Let the tool repair your file. 
5. When the tool finishes, select OK.
6. Go to the File menu.
7. Select Utilities and then choose Verify Data.
8. Let the tool check your file for data issues.

Once done, I'd recommend updating QBDT 2021 to its latest release. This is to make sure your software is up-to-date so you always have the latest features and fixes. You can refer to this article for the step-by-step guide: Update QuickBooks Desktop to the latest release.

After that, go to the Reports menu and select the Company & Financial option to pull up the Balance Sheet Prev Year Comparison report. Then, customize it to focus and filter the information you need.

Also, QBDT provides different reports that cater to your business needs. If you wish to learn more about how the system generates your reports, I'd recommend checking out this article: Understand reports. It also includes a table where it lists all the available reports in QBDT and their description.

Don't hesitate to let me know if there's anything else I can help you with. If you have other reports concerns or inquiries about managing your YTD transactions, I'm just around to help. Stay safe and take care always.