The pre-installed Samsung keyboard is already set as default, and new keyboards that are installed later must first be activated in the Keyboard Manager before they can be set as the default keyboard.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  General management
Step 3:  Tap on  Language and input
Step 4:  Tap on  On-screen keyboard
Step 5:  Tap on  Default keyboard
Step 6:  Tap on desired  keyboard
  1. Open the Settings
  2. Tap on General management
  3. Tap on Language and input
  4. Tap on On-screen keyboard
  5. Tap on Default keyboard
  6. Tap on desired keyboard

Samsung Instructions