the method `into_iter` exists for struct `String`, but its trait bounds were not satisfied

I wan't to concat name which is String with online_presence and requestor but I got this issue.

let concat: Vec<u8> = name
        .unwrap()
        .into_iter()
        .chain(online_presence.unwrap())
        .chain(requestor.to_keyed_vec(Default::default()))
        .chain(Self::env().block_timestamp().to_be_bytes())
        .collect();

error[E0599]: the method `into_iter` exists for struct `String`, but its trait bounds were not satisfied
   --> /Users/ganesholi/Developer/me-protocol/rusty-protocol-v0.1/contracts/providers/services/brands.rs:63:14
    |
61  |           let concat: Vec<u8> = name
    |  _______________________________-
62  | |             .unwrap()
63  | |             .into_iter()

Can somebody suggest do I resolve this issue?

Sounds a bit like the XY Problem.

If I understand correctly, what you really want to do seems to be to concat multiple strings together without occurring unnecessary allocations.

You'll at least need one allocation, at which point you can just format!. You can also use write! to pre-allocate a buffer.

let concat = format!("{}{}{}{}", name, online_presence.unwrap(), requestor.to_keyed_vec(Default::default()), Self::env().block_timestamp());

If you don't want a string, but instead want to chain together certain byte sequences use String::into_bytes and call into_iter + chain on that:

let concat = name.into_bytes().into_iter()
    .chain(online_presence.unwrap()) // assuming this already is an iter over bytes
    .chain(requestor.to_keyed_vec(Default::default())) // assuming this already is an iter over bytes
    .chain(Self::env().block_timestamp().to_be_bytes())
    .collect::<Vec<_>>();