PHP Import CSV Undefined offset

Sounds like the input file has some lines that dont quite fit.

You may need to do some checks before the insert like

$line_no = 1;

// Parse data from CSV file line by line
while(($line = fgetcsv($csvFile)) !== FALSE){
    $line_no++;
    if ( count($line) != 21 ) {  
        echo "$line_no needs to be looked at";
        continue;       // go to next iteration of the loop
    }
    // Get row data
    $postcode   = $line[0];
    $week1  = $line[1];
    $week2  = $line[2];
    $week3  = $line[3];
    $week4  = $line[4];
    $week5  = $line[5];
    $week6  = $line[6];
    $week7  = $line[7];
    $week8  = $line[8];
    $week9  = $line[19];
    $week10  = $line[10];
    $week11  = $line[11];
    $week12  = $line[12];
    $week13  = $line[13];
    $week14  = $line[14];
    $week15  = $line[15];
    $week16  = $line[16];
    $week17  = $line[17];
    $week18  = $line[18];
    $week19  = $line[19];
    $week20  = $line[20];
    
    $db->query("INSERT INTO nlbelevering 
            (Postcode, Week1, Week2, Week3, Week4, Week5, Week6, 
            Week7, Week8, Week9, Week10, Week11, Week12, Week13, 
            Week14, Week15, Week16, Week17, Week18, Week19, 
            Week20) 
    VALUES ('".$postcode."', '".$week1."', '".$week2."', 
            '".$week3."', '".$week4."', '".$week5."', 
            '".$week6."', '".$week7."', '".$week8."', 
            '".$week9."', '".$week10."', '".$week11."', 
            '".$week12."', '".$week13."', '".$week14."', 
            '".$week15."', '".$week16."', '".$week17."', 
            '".$week18."', '".$week19."', '".$week20."'");
    }
}

Of course you may consider checking all the lines in a pre parse of the file, and rejecting the whole file if any line turns out to have less than 21 columns. This all depends on the data, and if it makes sense to have a few lines missing from your database, or not.

BIG NOTE Your script is open to SQL Injection Attack. Even if you are escaping inputs, its not safe! You should consider using prepared parameterized statements in either the MYSQLI_ or PDO API's instead of concatenated values

Not only would prepared statements avoid the SQL Injections issues you could speed up the processing with a prepared statement as you only have to prepare (send to the database, compile and optimise) the query once instead of once per row of your input.

// prepare once outside the loop
$stmt = $db->prepare(
            "INSERT INTO nlbelevering 
                    (Postcode, Week1, Week2, Week3, Week4, Week5, Week6, 
                    Week7, Week8, Week9, Week10, Week11, Week12, Week13, 
                    Week14, Week15, Week16, Week17, Week18, Week19, Week20) 
            VALUES(?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)"
            );

while(($line = fgetcsv($csvFile)) !== FALSE){

    // bind new values each time round the loop and execute the query
    $stmt->bind_param('sssssssssssssssssssss',
                    $line[0], $line[1], $line[2], $line[3],
                    $line[4], $line[5], $line[6], $line[7],
                    $line[8], $line[19], $line[10], $line[11],
                    $line[12], $line[13], $line[14], $line[15],
                    $line[16], $line[17], $line[18], $line[19],line[20]
                );
    $stmt->execute();
}

UPDATE After seeing the CSV file.

CSV stands for "Comma Seperated Values" and therefore fgetcsv() assumes a file like

aaa,bbb,ccc,ddd,.......

If you have a "Semi Colon Seperated Value" file, you have to tell fgetcsv() to expect a different selerator, so from seeing you file change your code to use

// parmeter 2 need to be there to use param 3, 
// pick a number larger than any of the line, lenght or Newline will 
// denote a line.

// parameter 3 says to expect `;` as the seperator instead of a `,`

fgetcsv($csvFile, 1000, ";");