Not a DOI

The resolution service at https://doi.org resolves DOIs. You have attempted to resolve core/services/aop-cambridge-core/content/view/9A47A7486E6AA472446E1C1F9A939FC3/S1743921312019102a.pdf/the-aperiodic-variability-of-the-ultraluminous-x-ray-source-in-ngc-5408.pdf, which is not a DOI.

core/services/aop-cambridge-core/content/view/9A47A7486E6AA472446E1C1F9A939FC3/S1743921312019102a.pdf/the-aperiodic-variability-of-the-ultraluminous-x-ray-source-in-ngc-5408.pdf may be a Handle (DOIs are a special sort of Handle). If core/services/aop-cambridge-core/content/view/9A47A7486E6AA472446E1C1F9A939FC3/S1743921312019102a.pdf/the-aperiodic-variability-of-the-ultraluminous-x-ray-source-in-ngc-5408.pdf is a valid Handle you should be able to resolve it at https://hdl.handle.net.