Once you have converted the file to RGB/8, have Rasterised the layer to a (Pixel) layer, and have this correct layer selected, you should find the Plugin works as expected.
Although the standalone worked well, the plugin did not. I tried the new version (5.5.7) today with AP 1.8 on a scanned microscope slide. I recall that it used to work as a plugin as well as a standalone. To convert an (Image) layer to a (Pixel) layer, please right-click on the Layer in the Layers Studio and select Rasterise. I have had an older version of Astra Image for some time. Affinity Photo, an inexpensive direct rival to Photoshop with nearly. If this is showing as an (Image) layer and not a (Pixel) layer in the Layers Studio, this can also cause the Plugin not to be compatible. plugins from other sources, while others do not or have a limited capacity. In the Layers Studio, please ensure you have the correct pixel layer selected. If the menu option remains 'greyed out', this may be due to your active Layer selection. Please navigate to Document > Convert Format / ICC Profile. and change this document to RGB/8 and try using the Plugin once again. I am using Affinity Photo 1.8 (Windows 10) I cant use a Photoshop plugin SRDx.8bf. I have made over 4,000 images using these particular plugins in Photoshop, and I occasionally update them as well. In your first screenshot, I can see your Affinity document is using the CMYK colour space, I believe this needs to be RGB/8 to use with the Topaz Plugins. I cannot entertain the possibility of switching to Affinity Photo until my plugins are able to work with it. I can see that the Plugin is showing in this menu, though is currently 'greyed out' - which usually indicates the layer/document you are using is not compatible with the plugin.