Here is a complete guide to installing new memory including determining the type of memory required to locating the slots and inserting it. Heavily Crucial biased but that is not necessarily a bad thing.
The next to last line starts the physical installation tutorial.
Getting back to the OPs problem.
I suspect that the new memory has a configuration that the computer cannot handle. E.g., it is 4 chips of 64Mb each but the computer can’t handle that type and maybe can only deal with 8 chips of 32Mb each or some such weirdness.
When a computer comes out, they only set it up to handle memory chip configurations that are current and expected to come out soon. Of course, evedn larger chips come out and the memory makers then start making memory with the larger chips but with fewer chips on the card.
Sometimes a BIOS upgrade can fix it.