Hard errors can manifest as a memory module that does not register on start and prevents booting. They can also produce intermittent data corruption, in which case they can require multiple tests and reboots to differentiate them from soft errors.
When troubleshooting suspected hard errors, it is usually advisable to test RAM that is known to function at its default timings, which enables fast confirmation of hardware issues. RAM may be tested a stick at a time to find the faulty module then. If neither sets of RAM function correctly, you should verify that the timings are correct. Timings for RAM may be tried on default SPD settings, which can be researched by looking up a particular model or manufacturer’s recommendations. You can use a free memory testing programs, such as memtest86+, or the memory test provided with your operating system.
Sometimes RAM with hardware degradation or just not performing to specification can be made to work at slower settings. If the issue persists, however, it may be that the CPU or motherboard needs replacing or on the other hand, a BIOS/UEFI update may be required to better support the modules.