If you have PixelSquid, you need to upgrade it for Photoshop 2021. According to their website, . . .
We have redesigned the plugin to take advantage of new features available only within Photoshop 2021. Both the old and new plugins can run side-by-side. You will find the old plugin in the Windows | Extensions (Legacy) | PixelSquid menu in Photoshop 2021.
Important: While the plugins can run side-by-side, it is not recommended to use both of them in the same composition. The new plugin can read the layers created in the old plugin, but you cannot go backward – the old plugin cannot read the layers created in the new plugin.
You will find the installation instructions in the document below. NOTE: The new plugin is accessed via Plugins>PixelSquid.