Hi Hemlock,
Welcome to Affinity Forums :)
You have to click on one of the Pixel layers in the Layers panel to use the features/tools available in the Develop Persona.
When you open or drag a RAW file to Affinity, it will automatically open it the Develop Persona. This is where you should make the RAW conversion/development (i.e. all major global adjustments - generally speaking - to the image/raw data) before moving to the Photo Persona which is geared towards localised edits/compositing.
If the image you're opening it's not a RAW file but a generic jpeg (or another format) or you already processed the image trough the Develop Persona and moved to the Photo Persona there's no raw data to work with (a jpeg/png etc is an image that was already processed by the camera or other software). So to use the Develop Persona features/tools again you will have to pick a single layer. You will be affecting only that selected layer, not the whole image as with an original RAW file. This is the reason why it asks you to select an RGB pixel layer to work with.
