There can be only one account of detail type Exchange Gain or Loss.My invoice needs 4/5 individual lines but the message above comes up

It's great to see you in the Community, karynselfe127!

The error you got is right. We're unable to add more Gain/Loss account in the Chart of accounts.

On the other hand, even if you have more lines on your invoices, the realized gain or loss is based on the total invoice amount, not on the individual line amount.

This article has more information about reliased and unreliased gains or losses: Frequently Asked Questions about Home Currency Adjustments. It includes some reports where these gains or losses are posted.

Just comment below if you need more help about invoicing or about creating accounts.