prepared INSERT statement to get insert_id to use in a second prepared INSERT statement

Im trying to create my own register form but im having issues with prepared statements.

the idea is to create a prepared statement to insert info into the user table, then from that get the insert_id from the generated content to use in another insert statement

here is a version of my register script

  <?php

    $returnedId = '';

    include "includes/dbconnect.php";

    $stmt = $db->prepare("INSERT INTO `users`(`Username`, `Email`, `Password`) VALUES (?, ?, ?)");
    $stmt->bind_param('sss', $_POST['username'], $_POST['email'], $_POST['password']);
    $stmt->execute();
    $returnedId = $stmt->insert_id;
    $stmt->close();

    echo $returnedId;

    $allergystmt = $db->prepare("INSERT INTO 'user_allergy' ('user_id', 'allergy_id') VALUES (?, ?)");
    $allergystmt->bind_param('ss', $returnedId, $_POST['check_list']);
    $allergystmt->execute();
    $allergystmt->close();

    header('Location: index.php');

?>

the first prepared statement runs correctly and inserts information into the table, after that the $returnId variable is successfully echoed. next in the script is my second prepared statement, when it tries to run im getting the error that says:

Fatal error: Call to a member function bind_param() on a non-object in D:\filepath\register.php on line 17

it seems that my variable isnt being carried into the second prepared statement.

Your second query has syntax errors and failed to prepare. Since you have no error handling for database failures like this, your later code just blunders onwards:

$allergystmt = $db->prepare("INSERT INTO 'user_allergy' ('user_id', 'allergy_id') VALUES (?, ?)");
                                         ^---         ^--^---    ^-- etc...

You cannot use ' quotes on table and field names. ' indicate strings. None of those field/table names are reserved words, so there is NO need to quote them at at all:

$allergystmt = $db->prepare("INSERT INTO user_allergy (user_id, allergy_id) VALUES (?, ?)");
if (!$allergystmt) { die($dbh->errorInfo()); }

Note the addition of the errorInfo() output. Never assume a DB operation was successful. Always assume failure, and treat success as a pleasant surprise.