Hi @Antonio_Lacerda,
This was a known bug — when “Submit with Cmd+Enter” is enabled, the “Try Plan Mode” suggestion steals the Cmd+Enter keystroke and switches the mode instead of submitting your message.
A fix has already been shipped and will be available in the 3.1 stable release. Since you’re on 3.0.13, you don’t have it yet. In the meantime, you can switch to the nightly release track to get the fix now (note: after switching, you may need to restart Cursor or run Cmd+Shift+P > Developer: Reload Window for the keybinding change to take effect).
You can also follow this related thread where a staff member confirmed the fix: Cmd+enter forces plan mode.