One long beep followed by two short beeps on an ASUS motherboard means the RAM is not recognized. One or more RAM modules on the motherboard are not being recognized.
See:
https://www.thewindowsclub.com/computer-beep-codes-list
Typically on an old motherboard when that happens you examine the RAM modules before removing them to make sure you know which slots they are in and how they go in because you're going to put them back in those same slots after you finish cleaning them.
Follow these instructions to clean RAM modules:
https://computerinfobits.com/how-to-clean-ram-contacts/
After cleaning, insert RAM modules back into the exact slots you took them out. Never force them. Do make sure they are pushed into their slots after aligning them up correctly with their slot. They should not be partially pushed in.
See:
View: https://youtu.be/eNXIHCa6BOQ
If the problem persists after cleaning the RAM and properly reinserting them into the exact slots they should be in, then one of the RAM modules has likely failed.
Considering the age of the motherboard/computer, have you thought about a new computer?