There are two ways to achieve hierarchies:

1 Using Accounting groups:

a) debug

b) accounting > conifugration > financial reports > choose reports

c) enable the hierarchy option

d) Configuration > account groups

2) Using Reporting lines:

a) debug

b)  accounting > configuration > financial reports > Report Lines
c) Go to the line you wish to turn into a hierarchy

c.1) Put the type as foldable

c.2) Add all the children lines. make sure to define clear section names, codes, levels and sequences

c.3) Define the formula: usually you will want the balance of all the children lines.  the balance is computed using the Code of each child. The formula is shown on the next picture. 

d) Defining the children:

d.1) Define the Name, Code, sequence. make sure the level is the same for all children of the same level. !!! don't start the code of the section with a number or the formulas will not work.

d.2) Define formula:

        If this report line has children  lines too then use the method defined above.

        If the report line has no more children use:

                        balance = sum.balance : to calculate remaining balance of all remaining lines

                        credit = sum.credit: calculate remaining sum of all credits

                        debits = sum.debits :calculate remaining sum of all debits

d.3) Define the domain: [('account_id.code', '=like', '20%')]  this will take all the accounts that start with 20 . adapt as needed

d.4) group the remaining childs by id: account_id

Result: