Get the zero-based position of a field, given by its internal name in a content type given by its name. When the field does not exist in the content type, the return value is -1.

GetFieldPositionInContentType.ps1

param (
    [string]$ContentTypeName,
    [string]$FieldName
)

$result = -1
$currentPosition = -1

$ct = Get-PnPContentType -Identity $ContentTypeName 

if ($ct -ne $null)
{
     $fieldLinks = Get-PnPProperty -ClientObject $ct -Property FieldLinks

     foreach ($fieldLink in $fieldLinks)
     {
         $currentPosition++

         if ($fieldLink.Name -eq $FieldName)
         {
            $result = $currentPosition

            break
         }
     }
}

Write-Output $result