Exch. rate cannot be changed: Foreign cur. same as local cur.

What causes this issue?

The local currency is the same as the (header) foreign currency.
As it does not make sense to change the exchange rate in this case, the
system does not allow you to carry out this function.

How to fix this error?

If you wish to change the exchange rate, you have to change the (header)
foreign currency and then select the function again.

Error message extract from SAP system. Copyright SAP SE.