Powershell Script Finds First Result and Quits With Error

This stops on the first line where it finds the "@" and does not continue through the remainder of the text file. Also, when it does complete and gives me the first value that I am looking (120155) for and has an error I am not familiar with.

Error:
Cannot find an overload for "indexOf" and the argument count: "2".
At line:4 char:1
+ $end = $str.indexOf(",@", $start)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodException
    + FullyQualifiedErrorId : MethodCountCouldNotFindBest
 
Exception calling "Substring" with "2" argument(s): "startIndex cannot be larger than length of string.
Parameter name: startIndex"
At line:7 char:1
+ $result = $str.substring($start, $length)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : ArgumentOutOfRangeException
 
120155

PS H:\>

$str = Get-Content C:\Temp\Verify_Integrations\INT_integrations_post2.txt -Encoding Ascii
 
$start = $str.indexOf("@") + 1
$end = $str.indexOf(",@", $start)
$length = $end - $start
 
$result = $str.substring($start, $length)

$result