tried to use Fieldmanager_Grid for the first time, i've had no problems with the other fieldmanager classes
followed the demo here https://github.com/alleyinteractive/fieldmanager-demos/blob/master/lib/class-fm-demo-grid.php

        'table' => new Fieldmanager_Grid( array(
            'js_options' => array(
                'startRows'    => 6,
                'startCols'    => 5,
                'colHeaders'   => true,
                'rowHeaders'   => true,
                'minSpareCols' => 1,
                'minSpareRows' => 1,
                'contextMenu'  => true,
            )
        )),

First problem was 'grid.js?ver=1.2.6:88 Uncaught TypeError: $(...).live is not a function

using WP5.6 - Managed to work around that installing the plugin 'enable-jquery-migrate-helper'

However although this allowed the grid to display correctly i was unable to use the control to persist data to the post_meta