Like moonboots assumed correctly you have no image for the texture node. You need to create one.
- Press the "+ new" button to create an image. Give it a name
- Select the image in the image panel on the left side so you can see the image
- Save the image either with
Save as... or with
Save as... will save your image in a separate file. Alternatively, you can use the
Pack. This will pack your image into the .blend file.
Caution! Whatever you choose your image will not be saved when you save your blend file! You need to use the 'save image' (ALT + S) command individually. If you close Blender it will remind you when you have unsaved images. But if Blender crashes or freezes your unsaved changes of the image are lost.