virtualbox – Error: mount.nfs: requested NFS version or transport protocol is not supported

I have 2 virtual machines running on Virtualbox using the ubuntu 20.04 operating system. One machine is used as a server, the other is used as a client.
I have installed nfs-kernel-server for the server and nfs-common for the server and client respectively.
This is my /etc/exports file

When I run the command on the client:
sudo mount 10.0.2.15:/mnt/nfs_share /mnt/nfs_clientshare

I only received 1 line of notification:
mount.nfs: requested NFS version or transport protocol is not supported

Can anyone tell me where my mistake is and how to fix it? Thank you very much.

I have tried restarting nfs-kernel-server, firewall. But still cannot fix this error