Certificates need to contain keys of sufficient length. For a certificate based on the RSA algorithm, a key of 1024 bits is too short. Large certificate providers have announced that they will no longer issue certificates with RSA keys of up to 1024 bits as of 1 January. Some of these providers will revoke certificates with RSA keys of up to 1024 bits that they have issued previously.
Download NCSC factsheet from here - https://www.ncsc.nl/binaries/en/services/expertise-advice/knowledge-sharing/factsheets/factsheet-certificates-with-1024-bit-rsa-are-being-phased-out/1/Factsheet%2BCertificates%2Bwith%2B1024%2Bbit%2BRSA%2Bare%2Bbeing%2Bphased%2Bout.pdf