The Stock ROM will require to occupy the whole device.
So you more or less cannot flash a Stock ROM from a Custom Recovery!
So you are needing to search for the information to flash the whole Stock ROM. Odin is easier from someone else’s Windows … or …
https://github.com/Benjamin-Dobell/Heimdall
Do I have ROMs that don’t match my device?
Please be absolutely certain of this before you go further! Try
https://www.aida64.com/downloads/latesta64droid
Edit, or [HOWTO] Find device codename
Also @rockford I just remembered [HOWTO] (Needs expert review) Flash Galaxy s9+ to Stock Android 10 using heimdall-gui