Since the last post on this thread, I contacted QuickBooks support multiple times. Every single time, this is what happens:

1. I explain the problem that qb_taxform_link.xlt supplied with QB2021 is incompatible with 64 bit version of Office 2016. I also explain that I have contacted them many times about this before.
2. Yet, every time they insist that I must update/repair/reinstall QB 2021, update/repair/reinstall Excel before they can investigate/help.
3. I update/repair/reinstall Excel/QB to prove that was not the cause.
4. Then, they go through the routine suggestions, like - I should switch to 32 bit version of Excel, OR I should downgrade Excel to earlier version like 2006 or 2009 or 2013.
5. When I explain them why I can't move to older version or to 32 bit version, they refer my case to level 2 or level 3 support person.
6. Then, I don't hear from them for 2 months.
7. When I call them back to enquire about the case, the cycle starts all over again from the beginning at #1..

I am at a loss, and not sure what to do.