I tried exporting a datapack with the command and am seeing some errors in chat, some villager professions don't seem to have exported (such as the farmer):

[18:07:17] [Render thread/INFO]: [System] [CHAT] Could not find codec for factory class com.faboslav.friendsandfoes.util.TradeOffersUtil$BuyForOneEmeraldFactory
[18:07:17] [Render thread/INFO]: [System] [CHAT] Could not find codec for factory class net.mehvahdjukaar.supplementaries.common.entities.trades.AdventurerMapsHandler$RandomAdventureMapTrade
[18:07:17] [Render thread/INFO]: [System] [CHAT] Could not find codec for factory class com.teamremastered.endrem.events.ModEvents$EREyeTrade
[18:07:17] [Render thread/INFO]: [System] [CHAT] Could not find codec for factory class net.minecraftforge.common.BasicItemListing
[18:07:17] [Render thread/INFO]: [System] [CHAT] Could not find codec for factory class com.github.alexthe666.alexsmobs.misc.EmeraldsForItemsTrade
[18:07:17] [Render thread/INFO]: [System] [CHAT] Could not find codec for factory class net.mehvahdjukaar.moonlight.api.misc.ModItemListing
[18:07:17] [Render thread/INFO]: [System] [CHAT] Could not find codec for factory class de.teamlapen.vampirism.entity.villager.Trades$ItemsForSouls
[18:07:17] [Render thread/INFO]: [System] [CHAT] Could not find codec for factory class de.teamlapen.vampirism.entity.villager.Trades$ItemsForHeart

I have the following mods that seem to be relevant:

• Vampirism
• Friends and Foes
• Supplementaries

Custom trade types cannot be exported because the mod doesn't know what exactly has to be encoded and decoded.
What do you need the datapack for? If you only need to add trades, you can write a datapack that only contains the new trades in the desired groups, with every "replace" set to false, instead of replacing everything with the exported datapack. Alternatively, if those broken trades are not that important with you, you can export the datapack without those mods and then add them back.

From the front page:
As of 1.2.0, the mod now includes all modded trades in the default configuration. If you want to tweak the automatically generated trades, you can create a datapack with the current configuration using the "/dynamicvillagertrades export" command.
Perhaps this line is easy to misunderstand; I assumed it meant modded trades could be exported with the command!

My goal is to more or less redo the trades, so I should be able to use the exported vanilla trades as a base to work with.
Thank you for the information!

I updated the description. I'm also reopening this issue as there are definitely better ways to handle this problem than just not generating the JSON file altogether.