I believe this is a documented issue for Dynamics Nav, and there was a fix described in the link below. Very frustrating to run into this error today and realize I had to manually create a reversing entry throught a General Journal. Can BC team please fix this issue?

https://forum.mibuso.com/discussion/comment/295389#Comment_295389

Here are the steps we did to get an error message "You cannot reverse Vend. Ledger Entry No. (xxx) because the entry has an associated Realized Gain/Loss entry.":
1) Create Purchase invoice for vendor in foreign currency
2) Create and apply a payment to this invoice, also in foreign currency
3) Unapply the payment from this invoice
4) Attempt to "reverse transaction" on the payment