Causes of cracking in tack weld?

Please advise the cause of cracking in tack weld joint?
thicknesss 55 mm, material C-Mn
tack weld and root run is GTAW process

It looks like weldment contamination and thin tack weld.

Yes, too thin, one pass at root.

Preheat temperature and soak time?

preheat at 110 deg. C in one hour.

It may also an interruption of welding.
If your weld product was not completed a minimum 3 layers over the full thickness and length.