Tjasa is from the Slovenian Tjaša — Slovenian feminine name (variant of Tatjana). Tjasa is one of the Slovenian feminine names — central to traditional Slovenian Catholic heritage.
Tjasa does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Tjasa reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.