I actually tried multiple solutions but i don't know why it can't be positioned!? Here is the code:

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:smart_tank1/widgets/home_widget.dart';
import 'package:intl/intl.dart';


class HomeScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final Size size = MediaQuery.of(context).size;
    return Scaffold(
      body: SafeArea(
        child: SingleChildScrollView(
          child: Column(
            children: [
              Stack(
                clipBehavior: Clip.none,
                children: [
                  ClipPath(
                    clipper: DrawClip(),
                    child: Container(
                      height: size.height,
                      width: double.infinity,
                      decoration: BoxDecoration(
                          gradient: LinearGradient(
                              colors: [Color(0xff46ddbf), Color(0xff3088e2)],
                              begin: Alignment.topLeft,
                              end: Alignment.bottomLeft
                              )
                              ),
                      child: Column(
                        children: [
                          Container(
                    height: size.height,
                    width: double.infinity,
                    child: Column(
                      children: [
                        SizedBox(height: 40,),
                        Container(
                          height: 80,
                          width: 80,
                          child: CircleAvatar(
                              backgroundColor: Colors.white,
                              child: Icon(Icons.person, size: 40, color: Colors.grey,),
                            ),
                        ),
                      SizedBox(height: 40,),
                       Row(
                         children: [                     
                              Padding(
                                padding: const EdgeInsets.only(left: 30.0),
                                child: Text('Today', 
                                style: TextStyle(
                                  color: Colors.white,
                                  fontWeight: FontWeight.bold,
                                  fontSize: 20,
                                  ),
                                ),
                              ), 
                         ],
                       ), 
                       SizedBox(height: 10,),
                       Row(
                         children: [                     
                              Padding(
                                padding: const EdgeInsets.only(left: 30.0),
                                child: Text(
                                  DateFormat('dd-MM-yyy').format(DateTime.now()).toString(), 
                                style: TextStyle(
                                  color: Colors.white,
                                  fontWeight: FontWeight.bold,
                                  fontSize: 20,
                                  ),
                                ),
                              ), 
                         ],
                       ),
                      
                        
                      ],
                    ),
                  ),
               ],
                ),
                  ),
                  ),                   
                        SizedBox(
                             height: 200,
                             width: double.infinity,
                             child: Padding(
                               padding: const EdgeInsets.all(20),
                                child: Card(
                                  color: Colors.grey,
                                  shape: RoundedRectangleBorder(
                                      borderRadius: BorderRadius.circular(20),
                                    ),
                                  elevation: 5,
                                  child: Text('Here'),
                                ),
                             ),
                           ),
                      
                ],
              ),
            ],
          ),
        ),
      ),
    );
  }
}


class DrawClip extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    // TODO: implement getClip
    Path path = Path();
    path.addOval(
        Rect.fromCircle(center: Offset(size.width * 0.5, 50.0), radius: 300));
    return path;
  }

  @override
  bool shouldReclip(covariant CustomClipper<Path> oldClipper) {
    // TODO: implement shouldReclip
    return true;
  }
}
 

I have tried a lot of things like Positioned widget and SizedBox even i tried to seperate widgets and do the work out of the Column widget but i couldn't reach any result to make it work it didn't work and i don't why? everything seems to work fine but it doesn't, please help!

You can find what i'm trying to reach described in the picture below: