Posting area R001 for Payment on account

Hi, Experts!

I am trying to do Payment on account (in tcode FP05 for an item click buttons: Clarify Payment -> Post online -> On Account).

Error message appears: "Account determination not possible for key LE21  /      /01    /0060 /0010, posting area R001".

In my opinion:

- in Posting area R000 I define G/L account for Down payment (credit posting, amount with tax)

- in Posting area 0010 - G/L account for Tax on Sales (credit posting, tax)

  - in Posting area 0020 - G/L account for Offset tax (debit posting, tax)

  - also post (debit) on Bank account (amount with tax).

Why does Posting area R001 need?

If for tax determination, else which role does G/L account from this Posting area have?

Thanks in advance,

Dmitriy.