Computer cant ¨find¨ my ssd

Hello everyone

I am currently building a frankenstein-style computer out of old pc and laptop parts. Everything seems to be working fine, and the computer itself boots without issues. My problem is that I cannot access the BIOS startup, I simply get directed to the hardwire diagnostics when I try. All the tests pass except for the storage one - it doesnt even start, and tells me that I have no hard drive installed. I realise that I've probably installed the ssd incorrectly but no guide/video/picture installs it any different than I. Help would be greatly appriciated. Also, as you might notice, this is my first time posting here and on Reddit in general so very sorry for any mistakes in formatting and so on. English is not my first language but I hope my skills are sufficient. Thanks!