I have installed kubernetes on digital ocean cloud. I installed both flannel and calico as CNI. Will, that causes any problems in my cluster?
You can use them together, but make sure you configure things so that Calico isn't trying to control tunneling or routing. This joint configuration is sometimes called "Canal", but you can find the docs most on the Calico side at https://docs.projectcalico.org/v3.4/getting-started/kubernetes/installation/flannel
Calico and Flannel uses different default IP subnets and CNI driver binaries, they will not work together on the same cluster if you deploy them using standard (not Canal) installations. But it's required for Kubernetes cluster to have one of the network add-on installed. You are not limited to use Flannel or Calico add-ons, there are more of them
To remove Calico or Flannel from the cluster usually it's enough to run kubectl delete -f <calico-or-flannel.yaml> and reboot all nodes to get rid of interfaces created by Calico or Flannel. You may need to rejoin worker nodes to the cluster after that.