You may have added your work account to your Windows 10 home computer so that you could access work resources from home. And your organization may have imposed a strict PIN complexity requirement like “Must be at least 20 digits which may not occur anywhere in the decimal expansion of π.” But now, you’ve removed that work account from the PC (say, because you shifted to using a different PC as your your work-from-home primary computer), yet the system won’t let you switch back to a four-digit PIN. When you try to change the PIN, it tells you that it fails to meet the PIN requirements of the work account that you already removed.

What you need to do is remove the PIN entirely, and then re-add it.

For some reason, the PIN complexity requirements are remembered with the PIN, so changing the PIN will still need to conform to the complexity requirements. But if you remove it, and create a new PIN, then the new weaker requirements are used.

When you remove the PIN, you get a stern warning that this will invalidate your fingerprint and other credentials. And that’s my guess as to why the PIN complexity requirements are still enforced: Because the complex PIN is being used to protect some resources, and switching to a weaker PIN would leave those resources at greater risk. Instead, you have to delete the PIN (losing access to those resources entirely), and then create a new PIN (which will not have access to those sensitive resources).

Though it would be nice if they explained that and told you to delete the PIN and recreate it to avoid the old requirements.