Applicants for U.S. immigrant visas and K-1 (fiancée) visas are required to submit a set of civil documents in support of their application. This includes, for example, birth, marriage, and police certificates.
Effective June 15, 2021, the U.S. State Department has extended the validity of police certificates to 2 years. See 9 FAM 504.4-4(A) and NVC Immigrant Visa Process–Step 7. Collect Civil Documents.
Police certificates now expire after two years, unless the certificate was issued from your country of previous residence and you have not returned there since the police certificate was issued. The visa cannot be valid beyond a certificate’s expiration date.
For basic information about how to apply for a police certificate, see the State Department’s Reciprocity and Civil Documents by Country.