Value cannot be null.\r\nParameter name: clientRecipients

Santosh1986 
The error message is rather ambiguous here, but it actually means you don't have a valid recipient represented by tfids.

A working PowerShell example would be:

Invoke-RestMethod `
    -Method Post `
    -Uri "https://dev.azure.com/$orgName/$projectName/_apis/wit/sendmail?api-version=7.2-preview.1"    `
    -Headers @{
    Authorization  = "Bearer $accessToken"
    "Content-Type" = "application/json"
} `
    -Body (@{
        # requires at least one workitem id, integer
        ids     = @($wiid)
        fields  = @("System.Id")
        message = @{
            subject = $subject
            body    = $body
            to      = @{
                # if not provided, then clientRecipient missing error
                # note you can get the recipient id of your orgnization from:
                # https://vsaex.dev.azure.com/<your_org>/_apis/userentitlements?api-version=6.0-preview.3
                # just use a GET request with authorization header
                tfIds = @($recipientId)
            }
            # yes, both cc and replyTo are required
            # but you can use an empty object
            cc      = @{}
            replyTo = @{}
        }
    } | ConvertTo-Json -Depth 5)