Why are Proto-Germanic *taikijaną and Proto-Indo-European *deyḱ- cognate? I don't understand why are PGmc k and Proto-Indo-European ḱ cognate?
The *k might be explainable as the reflex of a Pre-Proto-Germanic geminate *-kk- that simplified to *k after a heavy syllable.
There is a disputed proposed sound law "Kluge's law" that derives geminates like -kk- from PIE sequences of plosive + *n (when the *n starts an accented syllable). I don't know whether there is any plausible source of *n in this word.
The alternative explanation of the source of geminate plosives in Germanic is "expressive lengthening".