Add Web Part Zones to default.aspx

Open the default.aspx page in edit mode, from inside the SharePoint Designer. (You may need to open the "Design" tab in SharePoint Designer.)

Select the lower row of the table.

Then, from the "Table" menu in SharePoint Designer, select "Insert" and then "Row Below".

Click into the row you just created. Add a table with 1 row and 1 column. You can create more rows or columns if needed. From the "Table" menu, select "Insert Table". Use the default parameters (or you can change if needed).

Insert >> SharePoint Controls >> Web Part Zone. Place the cursor in the right place and insert Web Part Zones.

Add the Web Part Zones to all cells needed.

Save the Page. Exit SharePoint Designer.

 View the Page in 'Edit page' mode from the Browser.