Step 1: see what has to change in the molecule.
Allyl alcohol is $\mathrm{CH_2{=}CH{-}CH_2OH}$. To reach propanal $\mathrm{CH_3CH_2CHO}$ we must do two separate jobs. First the carbon to carbon double bond has to disappear, and second the $\mathrm{-CH_2OH}$ at the end must be lifted to a $\mathrm{-CHO}$ without going further to an acid.
Step 2: remove the double bond.
Pass hydrogen over a palladium catalyst ($\mathrm{H_2/Pd}$). This adds hydrogen across the C=C and turns allyl alcohol into propan-1-ol, $\mathrm{CH_3CH_2CH_2OH}$, a saturated primary alcohol.
Step 3: oxidise gently to the aldehyde.
Now use a mild oxidising reagent that stops cleanly at the aldehyde stage, namely PCC (pyridinium chlorochromate). A strong oxidiser would overshoot and make the acid, but PCC is controlled and gives propanal.
Final answer: first $\mathrm{H_2/Pd}$ to reduce the double bond, then PCC to oxidise the primary alcohol to propanal.