From January 1st 2021, you can only use a PR card to apply if you haven't been granted settled status.

In order to apply for citizenship, applicants must have ILR, including Settled Status.
The application submission date is the relevant date for ALL practical purposes, not the biometrics appointment date.

If you have settled status, you CANNOT apply with a PR card from January 1st 2021.

Although it is possible to obtain settled status in less than five years in certain circumstances, for example, applying as the child (under 21) of a parent with settled status, it is still necessary to have lived in the UK for at least 5 years in order to apply for citizenship.

Scenario 1: If you are NOT married or Civil Partnered to a UK national

You can apply for citizenship with settled status 12 months after you first got permanent residence status.

If you were granted settled status as a child under 21, by linking your application to your parent's, you still need to have lived here 5 years and have had settled status for 12 months in the usual way.

You cannot apply with less than 5 years' residence, even if you were granted settled status more than 12 months ago.

From January 1st 2021, if you had PR before you got settled status, you can count that time towards the 12 month qualifying period. For example, if you came to the UK in 2013 and obtained PR status in 2018. In 2020, you applied for settled status. Usually, you'd need to wait 12 months after getting settled status, but because you had permanent residence for 2 years, you can apply straightaway.

You can still use your PR card to prove you acquired PR status and to show lawful residence when applying with settled status.

From December 2016, the covering letter sent with the PR card will have the date when you are deemed to have acquired PR, printed on the letter.

See: Date when PR status was acquired and PR card issue
Also: How to find out the date you acquired PR status.

If your PR document was issued at least 12 months ago, you don't need to find out the date when you acquired PR status for this purpose.

Scenario 2: If you ARE married or Civil Partnered to a UK national

You do not need to wait 12 months after being granted settled status to apply for citizenship.

Applicants need to be free from immigration time restrictions and have been in the UK for at least three years, but this doesn’t mean that three years is enough. As it takes at least 5 years to get PR or settled status, you still need to have been living in the UK for 5 years, before you can apply.

This is because the British Nationality Act was drafted in 1981, at a time when it was possible to become settled in less than five years. Now virtually all routes to settlement require at least five years’ residence.