Merhaba.

What does it mean: harikasız ?

I see this word quite often in colloquial texts on different websites.

harika is wonderful, awesome; I know it...

But -siz generally indicates that the feature is missing, so logically it should be like "wonderless" But I saw this word used positive contexts rather than negative... Perhaps it is a short for harikasınız?

It doesnt mean anything. It must be harikasınız.

But Harika can be the name of a person. Than Harikasız means without Harika.

yes, you´re right. "harika" means "wonderful, amazing, excellent,.."

It is not meaningful. It may have been written wrong. I have seen nowhere that "harikasız" is the abbrevation of "harikasınız". And it´s impossible according to the grammatical rules. 

addition:

Perhaps, it may hinge on regional dialects. For example: -sınız may be pronunced as -sığız in some villages of my city (Mersin, Southern Anatolia).