How to fix Data Guard cannot be created when standard database service port (1521) is blocked?

Summary:

How to fix Data Guard cannot be created when standard database service port (1521) is blocked?

Content (required):

Data Guard Association cannot be created when standard database service port (1521) is blocked for instances 

in Subnet: ocid1.subnet.oc1.af-johannesburg-1.aaaaaaaadb3saaclzveztv36unlhrk6q462wikr32v5ubvmhjl3sz3hsqepa by security rules 

associated with Subnet: ocid1.subnet.oc1.af-johannesburg-1.aaaaaaaalzlek6smov2axgxomgwzf3fglyz23prgn34thpqpwniwzzelnuhq

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