Problems updating a variable

You are declaring "tagUID" in a class, so you should not be using the @State property wrapper (which is for SwiftUI).

Try removing the "@State"... just use:

var tagUID = "UID"

Or since your NFCReader is an ObservableObject, you may mean:

@Published var tagUID = "UID"

The problem now, is that when you activate your NFCReader, it does not immediately return the tag...
...so you can not immediately assign reader.tagUID

To solve this, in NFCReader we can "Publish" the tagCode...
...and in the View, we show this published value.
Then, when NFCReader gets the tagCode, it will automatically update your SwiftUI View.

Try this:

import SwiftUI
import CoreNFC

struct ContentView: View {
    @StateObject private var reader = NFCReader()

    var body: some View {
        VStack{
            Text(reader.tagUID)
            Button {
                reader.activate()
            } label: {
                Text("Scan")
                .font(.title)
                
            }.padding()
            
        }
    }
}

class NFCReader: NSObject, ObservableObject, NFCTagReaderSessionDelegate {
    @Published var tagUID = "UID"
    var session: NFCTagReaderSession?
    
    func activate() {
        self.session = NFCTagReaderSession(pollingOption: .iso14443, delegate: self)
        self.session?.alertMessage = "Hold Your Phone Near the NFC Tag"
        self.session?.begin()
    }
    
    func tagReaderSessionDidBecomeActive(_ session: NFCTagReaderSession) {
        print("Session Begun!")
    }
    
    func tagReaderSession(_ session: NFCTagReaderSession, didInvalidateWithError error: Error) {
        print("Error with Launching Session")
    }
    
    func tagReaderSession(_ session: NFCTagReaderSession, didDetect tags: [NFCTag]) {
        print("Connecting To Tag")
        if tags.count > 1{
            session.alertMessage = "More Than One Tag Detected, Please try again"
            session.invalidate()
        }
        let tag = tags.first!
        session.connect(to: tag) { [self] (error) in
            if nil != error{
                session.invalidate(errorMessage: "Connection Failed")
            }
            if case let .miFare(sTag) = tag{
                let UID = sTag.identifier.map{ String(format: "%.2hhx", $0)}.joined()
                print("UID: \(UID)")
                print(sTag.identifier)
                session.alertMessage = "UID Captured"
                session.invalidate()
                DispatchQueue.main.async {
                    self.tagUID = "\(UID)"
                }
            }
        }
    }
}

Did that help you, @00ri?