The suppliers entered are not valid sources of supply

What causes this issue?

The source of supply check found that neither the external vendor &V1&
nor the internal vendor &V2& entered are valid sources of supply.
The source of supply key for the material is "&V4&".

System Response

If both an internal and external vendor are entered, the sales price
calculation function checks only whether the vendor entered is valid;
it does not search for a valid vendor.
No calculation could be performed for material &V3&.

How to fix this error?

If necessary, repeat the calculation without entering a vendor. The
system may be able to determine another valid vendor for material &V3&.
Alternatively,check the settings that are relevant to supply source
determination, such as the source of supply key, the purchasing info
records for external procurement, the supplying sites for internal
procurement, and so on.
Links for checking the master data and Customizing settings:
<DS:TRAN.MM43>Display Material Master Data</>
<DS:TRAN.ME1M>Info Records - List Display</>
<DS:TRAS.WB03>Display Plant</>
<DS:TRAN.MK03>Display Vendor</>
<DS:SIMG.SIMG_CFMENUSAPCOMKJ>Reference Purchasing Organization</>

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.