"WhatsApp messages on computer without a mobile phone" , install anbox if you r on linux , you can also go for androidx86 install in virtualbox if you want the full experience and you can even install a full os like remix os on either virtualbox, or in dual boot(i recommend you anbox,cause you just need to use whatsapp, you can find many manuals on web on how to install it )

neverware was going in the right direction of implementing a os which can handle linux and android apps natively , but their google-partnership killed it(i maybe a bit wrong here)

but if you are on windows , i recommend MEmu android emulator, i have tried many others , but it is the best , out of them

**you can try signal or telegram , both are great , and have full cross-platform support

note: whatsapp's multi device is actually a feature in development(may take a lot time) and you can check about it on wabetainfo.com , also a whatsapp account can only be used on one single android device (means , if you install whatsapp on anbox or something else , you wont be able to access the same account on your phone & vice-versa)