Certificate check failed

I have changed TLS_PEM_CA.Using the cacert.pem to fill TLS_PEM_CA .But it failed.I change the debug_level from 0 to 1.
Something return like this.

ssl_tls.c:4643: |1| x509_verify_cert() returned -9984 (-0x2700)

mbedtls_ssl_handshake() returned -0x2700
How i configure it.thanks for your help.

Start with the TLS_PEM_CA and cacert.pem configuration described in the issue, then inspect the reported failure at ssl_tls.c:4643 around x509_verify_cert() and mbedtls_ssl_handshake(). Reproduce the -0x2700 result with debug_level 1 and trace the certificate-verification setup; done means the required configuration is identified and the handshake succeeds or the remaining failure is documented.

Written by the indexing model from the issue text.