How to remove misspelled words on my Galaxy S8+

There’s no personal dictionary that stores all the learnt words in the Samsung Keyboard. Or maybe there's one but users are not given access to it to do any changes.
Anyway, you can delete misspelled words with the following methods:

1. Long tap the misspelled word that appears in the predictive bar, then you will get a prompt to delete this word.

2. You can also go the Samsung keyboard settings, tap Reset to default settings > Clear personalised data. Just note that by doing so, it will delete all the learnt words including the correct ones.