What’s your OS, Obsidian version, etc? If you type cmd/ctrl + p, then type Show debug info, and paste the results here, that will give folks some info about your system. It looks like this:
SYSTEM INFO:
Obsidian version: v1.5.8
Installer version: v1.5.8
// more
On macOS, spellcheck is on by default for new vaults and uses the system spellchecker.