macOS will not allow screen capture or remote input until you grant two permissions by hand. It asks once, and a refusal is remembered.

Enabling permissions

Open System Settings → Privacy & Security and grant SullTec Remote both of the following. They are separate permissions and one without the other produces a session that half works.

• Screen & System Audio Recording — without it the session connects and shows a black screen.
• Accessibility — without it the screen is visible but keyboard and mouse do nothing.

Quit and reopen SullTec Remote after granting them; macOS applies the change to a process only when it next starts. If the app is not listed, start a session first — macOS lists an application once it has asked.