$entry_id is just an integer, the ID of the entry that was updated. The $entry array that contains all the field values isn’t passed to the gform_after_update_entry hook.
You’ll need to update the update_team function to get the entry before you define the $TeamName and $RiderPK variables, e.g.
Okay next (related) question. If I delete the entry, I need to send an update back again to revert the team member status back to “unassigned”. I’m not able to get this one working. Can you spot my issue?
***/Revert member back to unassigned after deletion from a team./***
add_action( "gform_delete_entry_18", "revertback", 10, 1 );
Function revertback($form, $entry_id){
global $wpdb;
$entry = GFAPI::get_entry( $entry_id);
$RiderPK = $entry["35"];
$PriorStat = $entry["36"];
$wpdb->update( 'RegisteredMembers', array( 'Program' => 'Unassigned', 'TeamStatus' => $PriorStat), array( 'submission_id' => $RiderPK,));