I've replaced the "Isolated" parameter with "isolated" then it worked.
<?xml version="1.0" encoding="UTF-8"?>
<vmext:VMWExternalNetwork
xmlns:vmext="http://www.vmware.com/vcloud/extension/v1.5"
xmlns:vcloud="http://www.vmware.com/vcloud/v1.5"
name="example-extnet"
type="application/vnd.vmware.admin.vmwexternalnet+xml">
<vcloud:Description>ExternalNet</vcloud:Description>
<vcloud:Configuration>
<vcloud:IpScopes>
<vcloud:IpScope>
<vcloud:IsInherited>false</vcloud:IsInherited>
<vcloud:Gateway>172.20.x.x</vcloud:Gateway>
<vcloud:Netmask>255.255.0.0</vcloud:Netmask>
<vcloud:Dns1>172.20.x.x</vcloud:Dns1>
<vcloud:Dns2>172.20.x.x</vcloud:Dns2>
<vcloud:DnsSuffix>apro.com.tr</vcloud:DnsSuffix>
<vcloud:IpRanges>
<vcloud:IpRange>
<vcloud:StartAddress>172.20.160.100</vcloud:StartAddress>
<vcloud:EndAddress>172.20.160.200</vcloud:EndAddress>
</vcloud:IpRange>
</vcloud:IpRanges>
</vcloud:IpScope>
</vcloud:IpScopes>
<vcloud:FenceMode>isolated</vcloud:FenceMode>
</vcloud:Configuration>
<vmext:VimPortGroupRefs>
<vmext:VimObjectRef>
<vmext:VimServerRef
href="https://vcdip/api/admin/extension/vimServer/vcenterid" />
<vmext:MoRef>dvportgroup-28</vmext:MoRef>
<vmext:VimObjectType>DV_PORTGROUP</vmext:VimObjectType>
</vmext:VimObjectRef>
</vmext:VimPortGroupRefs>
</vmext:VMWExternalNetwork>