TypeError: Cannot assign to read only property ‘linkCaption’ of object ‘#’

Why it is not possible assign value to an object here in TypeScript?

let postList2 = { ...postList };
let p = postList2[selectedPostId];
p.linkCaption = e.target.value; // <-- here
postList2[selectedPostId] = p;
dispatch(setReadyMadePostCampaign(postList2));

I use let, TypeScript check does not find any syntactical error.