For a list given by its name and for a view in this list given by its name, get the zero-based position of the field given by its internal name. When the field does not exist in the view, the result is -1.

GetFieldPositionInView.ps1

param (
    [string]$Listname,
    [string]$Viewname,
    [string]$Fieldname
)

$result = -1
$currentPosition = -1

$view = Get-PnPView -List $Listname -Identity $Viewname -Includes ViewFields

if ($view -ne $null)
{
    $viewFields = $view.ViewFields

    foreach ($viewField in $viewFields)
    {
         $currentPosition++

         if ($viewField -eq $Fieldname)
         {
            $result = $currentPosition

            break
         }
    }
}

Write-Output $result