I am using self signed cert for keycloak. but other application encounter cert issue

my keycloak cert is signed by a ca cert (which is also created by myself)
when other application try to login via openid connect, always throw exception:
OpenIDConnect::Discovery::DiscoveryFailed (SSL_connect returned=1 errno=0 state=error: certificate verify failed (self signed certificate in certificate chain)):

is there a good way to generate free cert for testing ?

thanks for advice.
this is a test environment. the domain is fake. I cannot use Let’s Encrypt to issue a cert