That's great!
Either you just need a little longer Pause at the beginning to allow the display to power up or, more likely, the clear screen command takes a little longer to execute than you are giving it.
You can increase the timedelay for command with DEFINE LCD_COMMANDUS 2000, for example.

Once you have a couple of projects under your belt you get a feeling of what might be the problem when something doesn't work. I'd say that having analog functions switched on pins you want to use as digital is by far the most common issue people stumble across around here, followed by read-modify-write issues which sometimes is caused by analog functions not being switched off in the first place.

Keep it up and welcome to the forum!
/Henrik.