After indentation, your code look like:
<?php
echo "<!DOCTYPE html>";
echo "<html>";
echo "<head>";
echo "<title> Psychiatric disorders database </title>";
echo "<style>";
echo "<table align='center' border='1px' width:'600px' line-height:'40px;'>";
echo "</style>";
echo "</head>";
echo "<body>";
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "mydb";
include("connection.php");
$sql = "SELECT * FROM `schizophrenia raw datasets` ";
if($result = mysqli_query($conn, $sql)){
echo "<table>";
echo "<tr>";
echo "<th>Accession ID</th>";
echo "<th>Title</th>";
echo "<th>Abstract</th>";
echo "<th>Sample size</th>";
echo "<th>Tissue type</th>";
echo "<th>PMID</th>";
echo "<th>Platform ID</th>";
echo "<th>Platform details</th>";
echo "<th>Raw data</th>";
echo "</tr>";
while($row = mysqli_fetch_array($result)){
echo "<tr>";
echo "<td>" . $row['Accession ID'] . "</td>";
echo "<td>" . $row['Title'] . "</td>";
echo "<td>" . $row['Abstract'] . "</td>";
echo "<td>" . $row['Sample size'] . "</td>";
echo "<td>" . $row['Tissue type'] . "</td>";
echo "<td>" . $row['PMID'] . "</td>";
echo "<td>" . $row['Platform ID'] . "</td>";
echo "<td>" . $row['Platform details'] . "</td>";
echo "<td>" . $row['Raw data'] . "</td>";
echo "</tr>";
}
echo "</table>";
mysqli_close($conn);
echo "</body>";
echo "</html>";
?>
a } is missing.
Advice: Learn to indent properly your code, it show its structure and it helps reading and understanding. It also helps spotting structures mistakes.
Indentation style - Wikipedia[
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