You need to render at least 2 passes to use the denoiser.
This warning appears if the denoising option was enabled, however not enough passes were rendered for it to work. Denoiser needs at least 2 passes to work.
- Rendering was stopped too early by the user
- Rendering was stopped too early by the time/pass/noise limit
- Wait until at least 2 passes are rendered
- Increase or disable the progressive rendering limits
- Use the "resume rendering" option to render more passes