iperf3: error - unable to connect to server: Connection refused

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# Context

* Version of iperf3: iperf 3.7 (cJSON 1.5.2)

* Hardware: Intel CPU > i5-9300, Memory > 16G

* Operating system (and distribution, if any): Windows 10 and running Ubuntu 20.04 LTS(WSL 2)

_Please note: iperf3 is supported on Linux, FreeBSD, and macOS.
Support may be provided on a best-effort basis to other UNIX-like
platforms. We cannot provide support for building and/or running
iperf3 on Windows, iOS, or Android._

* Other relevant information (for example, non-default compilers,
libraries, cross-compiling, etc.):

_Please fill out one of the "Bug Report" or "Enhancement Request"
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# Bug Report

Now I'm running iperf3 on two machine. and Two machines are running windows 10, WSL 2, Ubuntu 20.04.
iPerf3 version has same both server and client machine. iperf 3.7

Two machine is possible icmp request and reply without any problem.
But when I tried to start iperf3 from client I met this error.

1) iPerf3 Server screen(Server IP is 192.168.200.253)
root@SHINCH01-15U560:/usr/bin# iperf3 -s

Server listening on 5201

2) iPerf Client Screen.(Client IP is 192.168.200.251)
root@CHANGHEESHIN01-5501I5:/usr/bin# iperf3 -c 192.168.200.253 -i 2
**iperf3: error - unable to connect to server: Connection refused**

I already shutdown windows firewall and ubuntu firewall with the following command

root@CHANGHEESHIN01-5501I5:/usr/bin# ufw disable
Firewall stopped and disabled on system startup
root@CHANGHEESHIN01-5501I5:/usr/bin# ufw status
Status: inactive

Can I know what is root cause?

Thank you.