Method invocation failed because [System.Object[]] doesn’t contain a method named ‘GetIPProperties’.
At :line:3 char:27
+ $PrimaryNIC.GetIPProperties <<<< ().UnicastAddresses[0].Address.IPAddressToString

if I replace your last line with:

foreach ($nic in $PrimaryNIC){
$nic.GetIPProperties()
}

I get:

Object reference not set to an instance of an object.
At :line:0 char:0

I have attempted some debugging of my own but have run to the end of my knowledge of PowerShell.

PS>[system.Net.NetworkInformation.NetworkInterface]::GetAllNetworkInterfaces() |
Where-Object {$_.GetIPProperties().GatewayAddresses} |
ForEach-Object {$_.GetIPProperties().UnicastAddresses|
Foreach-object{$_.Address.IPAddressToString}
}

I added the second foreach loop b/c my systems have both an IPv4 & IPv6 address

~Glenn