en_tn_condensed/act/13/35.md

This is why he also says in another Psalm

Paul's audience would have understood that this Psalm refers to the Messiah. Alternate translation: "In another Psalm of David, he also says about the Messiah" (See: rc://en/ta/man/translate/figs-explicit)

he also says

"David also says." David is the author of Psalm 16 from which this quotation is taken.

You will not allow your Holy One to see decay

The phrase "see decay" is a metonym for "decay." Alternate translation: "You will not allow the body of your Holy One to rot" (See: rc://en/ta/man/translate/figs-metonymy)

You will not allow

David is speaking to God here.