If you have the Classic Editor plugin or sometimes edit post types that use the Classic editor (ie. WooCommerce products), you can find metabox visibility settings in the Screen Options drawer at the top of the post editing screen.
First, locate the Screen Options tab at the top of the post editing screen.
Next, click the Screen Options tab to reveal your personalized settings.
Finally, under the Boxes section, enable the Completionist Tasks metabox by ticking its checkbox.
Step 1: Show the sidebar
If your Gutenberg editor sidebar is hidden, click the gear icon in the top-right of the screen to reveal it.
Step 2: Open additional editor options
Next to the gear icon, click the ellipsis in the top-right corner of the Gutenberg editor screen to reveal additional settings.
Click Options toward the bottom of the list to access personalized screen options.
Step 3: Enable the Tasks metabox
In the Gutenberg editor screen options pane, enable Completionist’s Tasks metabox by ticking its checkbox in the Advanced Panels section.
The Tasks metabox will then be visible toward the bottom of the Gutenberg editor’s sidebar.