Check your settings. Settings->General Management->Language and Keyboard->Samsung Keyboard settings (Or whatever keyboard that you use). Make sure Predictive Text is toggled on and Auto Replace if you want that.
Edit: If you use a Bluetooth Keyboard you will not have Predictive Text unless you use a 3rd party keyboard like SwiftKey and enable it.
Galaxy Fold 5 512g(VZW), Galaxy Book (original, VZW), Tab S8 Ultra, Tab S4, Galaxy Watch6Classic 47mm(VZW), Galaxy Watch5Pro(VZW), M8 Smart Monitor
Tab S8 Ultra book keyboard spell checking still broken. Also dark mode spell check with the soft keyboard dosen't work well with black on black font. There was a time when grey on black gave a passable result. Light mode has to be used for spell correction to see options. No auto captialisation, no grammar corrections. This was typed on the book cover keyboard with no assistance. Very poor that Samsung takes so long to fix their breakages.