The illocutionary act is a speech act: something that the speaker does by speaking. It often expresses an intention that the world matches what the speaker says—that their assertions are accurate, their promises sincere, their commands obeyed. But it doesn’t guarantee that the intention is realised. Illocutionary acts are usually defined in categories.
The illocutionary force is how your speech act ends up changing the world. Your intention as a speaker is that the illocutionary force matches the illocutionary act; but if the act misfires, it might not.