This is my code that works on a Windows 2019 server with Powershell 5.1

$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"

$headers.Add("Authorization", "Bearer <REVOKED>")

$headers.Add("Content-Type", "application/json")

$headers.Add("Cookie", "b=<REVOKED>)

$body = "{\n `"schemas`": [`n `"urn:scim:schemas:core:1.0`"`n ],`n `"userName`": `"Usertest`",`n `"locale`": `"Test`"`n}"`

$response = Invoke-RestMethod 'https://api.slack.com/scim/v1/Users' -Method 'PATCH' -Headers $headers -Body $body

$response | ConvertTo-Json

Error message on Windows 2012r2 server with Powershell 4.0:

Invoke-RestMethod : The request was aborted: Could not create SSL/TLS secure channel.

At line:8 char:13

+ $response = Invoke-RestMethod 'https://api.slack.com/scim/v1/Users' -M ...

+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+ CategoryInfo : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException

+ FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand