Widgets in Report can only handle one currency per widget. If you are receiving a mismatch currency error then two or more currencies are included in the widget.
To fix the error, remove accounts/campaigns until only one currency is included.
Don't know what campaigns/ad accounts are in a different currency? Use a table widget to look at the different currencies per entity (account/campaign etc).
If you are interested in converting currencies, we have a customized solution available. Please reach out to your customer success manager or firstname.lastname@example.org to learn your options for currency conversion.