You cannot assign bus. partner &1 because the partner is a natural person

What causes this issue?

You have chosen the business partner &V1&. However, this business
partner is a natural person.
In Treasury and Risk Management, only organizations and groups are
permitted as business partners in financial transactions.
Note:
Business-to-Business (B2B) in <LS>Treasury and Risk Management</> (TRM)
The financial transactions portrayed in TRM are B2B transactions between
your company and banks, financial institutions, brokers, or similar
institutions. Likewise, the master data required for the processes in
TRM relate to companies and financial institutions. Consequently, the
simplified blocking and deletion of personal data using <LS>SAP
Information Lifecycle Management</> (SAP ILM) is not implemented.
IF &[SWITCH]S4H_DOCU_CLOUD& = 'X'.
You must not portray financial transactions with natural persons in
<LS>Treasury and Risk Management</>.
ELSE.
If you use <LS>Treasury and Risk Management</> to portray financial
transactions with natural persons, for example, or your usage of
<LS>Treasury and Risk Management</> involves natural persons in other
ways, you need to deploy separate supplementary technical and
organizational measures to ensure that you respect the deadlines
governing the storage and deletion of personal data.
ENDIF.

System Response

The system does not allow you to assign a person.

How to fix this error?

Choose an organization or a group as the business partner.
IF &[SWITCH]S4H_DOCU_CLOUD& = ' '.


&CUSTOMIZING&
You can define when this system message is issued depending on your
requirements.
You do this in Customizing for <LS>Treasury and Risk Management</> under
<LS>Transaction Manager -> General Settings -> Tools -> Configurable
Messages -> </><DS:SIMG.OBA5>Change Message Control</>, using the
following:
Application area: <NP>TI</>
Number: <NP>031</>
ENDIF.

Error message extract from SAP system. Copyright SAP SE.