This is a documented issue for both NAV and BC:

BC:
https://experience.dynamics.com/ideas/idea/?ideaid=21c094ff-ce62-e911-b048-0003ff68b02f

NAV:

Error 1:
https://forum.mibuso.com/discussion/62534/error-msg-quot-you-cannot-reverse-cust-ledger-entry-quot

Error 2:
https://support.microsoft.com/es-pa/help/3058000/cumulative-update-7-for-microsoft-dynamics-nav-2015-build-40938

Error 3:
https://support.microsoft.com/hr-ba/help/2804303/you-cannot-unapply-an-entry-that-contains-the-unrealized-value-added-t

This happens when you post a purchase/sell order with one exchange rate and then post the payment on a different day with another exchange rate, the system creates the entries for gain/loss.

When you try to make a reverse first of all you need to unapply the payment, then when you try to make the reversal of the G/L Entries you get the error:

"It's not possible to reverthe the movement (##) because it has a gain/loss movement associated"

It also happen something similar when you apply a payment to a purchase order or sales order, when you unapply it and try to reverse the G/L entries the message pops out:

"It's not possible to reverse the movement (##) because it has a Unrealized VAT movement associated"

Already reported the issue to MS via support, and the only solution was: Reverse it manually, we doesn't have the solution for this issue in our roadmap.

It's a basic functionallity in every other ERP, when you need to reverse something and you click on the functions made to reverse an entry, it should REVERSE THE ENTRY.