Si ocurre algún problema, avísame enseguida.

Because after si meaning if, Spanish normally uses the present indicative for real or possible future situations:

• Si ocurre algún problema... = If any problem happens...

English often uses a present form here too: If a problem happens, let me know.

Spanish does not use the subjunctive after si in this kind of sentence. So:

• Si ocurre... ✅
• Si ocurra... ❌

A good rule: after si for a real condition, use the indicative.