From January 1st, if you have settled status, you MUST apply for citizenship with settled status.

In order to apply for citizenship, applicants must have ILR, including Settled Status.

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

You can still use your PR card to prove lawful residence, in the same way as you'd provide any other supporting documents for this purpose, such as P60s, self-assessments, etc.

If you haven't had settled status for 12 months and have a PR card, you can still rely on your PR status to apply before you've had settled status for 12 months, as noted here: When can I apply for citizenship?

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.

If you haven't been granted settled status, you can use your PR card until the end of June.

You need to have settled status when you submit the application, even if you are married to a British spouse.

You can't apply for citizenship with pre-settled status.

PR certification (only acceptable if you haven't been granted setled status), settled status or ILR is STILL REQUIRED even if you apply based on marriage to a British national. The rule applies to everyone.

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.

Citizenship applications can take up to 6 months, sometimes longer, and it can take a few more months to have your ceremony. As you are not British until after the ceremony, if you haven't got settled status, you should apply for it before the end of June, while awaiting a decision.

You cannot stay in the UK after 30 June 2021 while you wait for citizenship, unless you have ILR, settled or pre-settled status.

You can use your PR card number to apply for settled status without needing additional evidence of residence, see: Applying with PR

If you are applying with a PR document, make sure you have the right document

Citizenship applications are very costly the fee is not refunded if your application is refused, so it pays to make sure you are sending the right documents.

The most common cause for refusals has been applying with the wrong document, usually a Registration Certificate instead of a PR Card.

Registration Certificates and other residence documents can look similar to the PR certification card, you must ensure you’ve got the right document. The vignette will indicate the relevant document, not the blue card it is in.

If you have a blue cardholder, you need to look inside to make sure there is a PR card in there and not a registration certificate or another residence document. All the blue cardholders look pretty much the same from the outside.
The document on the left is a Registration Certificate, NOT ACCEPTABLE for citizenship.
The document on the right certifies Permanent Residence, and is ACCEPTABLE for citizenship (from January 2021, only if you have not been granted settled status.
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Non EEA family members get a pink biometric card. All pink biometric cards look much the same, so it is important to make sure it says PERMANENT RESIDENCE under type of permit, as shown below.
If it says anything else, such as FAMILY MEMBER, EU RESIDENCE, then it is NOT a PR card and cannot be used.

Non EEA nationals who are here under a different route other than as family members of EEA nationals are issued with BRPs (Biometric Residence Permits); not all BRPs show the holder has Indefinite Leave to Remain (ILR), it is important to make sure the document does show SETTLEMENT/INDEFINITE LEAVE TO REMAIN as below:

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Most EEA nationals wouldn't have a BRP unless they applied for ILR under the Windrush Scheme.
If you are in any doubt whatsoever as to whether you have the right document, please post up on the forum where the admins and lawyers can tell you whether you have the right thing before submitting your application.