Learn Dutch: Kan pas om half twaalf een datum prikken. - Can only set a date at half past eleven.

This is an example of "topic drop" in casual speech. The speaker dropped the subject Ik from the very beginning of the sentence. The full version is Ik kan pas..., but omitting an unstressed first word is very common in informal Dutch messages and conversations.

I know prikken usually means "to poke" or "to sting". How does it mean "to set" here?

The phrase een datum prikken is a standard Dutch idiom for "setting a date" or "scheduling a meeting". You can think of it as literally "pinning" a date on a physical calendar.

Wait, isn't half twaalf 12:30?

No, it is 11:30! This is a classic trap that still catches advanced learners. In Dutch, half-hours look forward to the next hour. So half twaalf literally means "halfway to twelve", which is 11:30.

Could I use alleen instead of pas to say I can "only" set a date then?

No, you must use pas. When "only" refers to time and means "not until", Dutch always uses pas. You would use alleen if you meant "exclusively", like if you only had one single date available.