Link to affected article: https://help.minecraft.net/hc/en-us/articles/4409236107789-Troubleshooting-Unable-to-Join-Multiplayer-Worlds-in-Minecraft

Although "Troubleshooting Unable To Join Multiplayer Worlds In Minecraft" works on most platforms, especially players of Nintendo Switch who has Super Mario Mash-up Pack could be baffled with the unlawful restriction (aka. "Platform restriction") and be confused as why, although the pop-up tells them, some players like those of unexperienced can be confused as why it was platform restricted after purchase when there were no sign of it if they did not experience the game (relates to some part of WEB-6914).

Proof of the issue: As of 1.20.81 on Nintendo Switch, Super Mario Mash-up Pack (including skins, resource pack and the world (template)) stifles cross-platform multiplayer and only allows Nintendo Switch Specific multiplayer, and when accessed the custom world with the resource pack from Super Mario Mash-up Pack on it, the world will be cross-platform multiplayer forcefully disabled until that world was accessed again with the resource pack removed. But in this site, there are no workaround of if multiplayer can't be used due to unlawful restriction. Not to mention, based of research, there are also other platform specific Marketplace ("Minecraft Store" on PlayStation) / DLC contents, including optional LittleBigPlanet Mash-up that's "platform specific" to PlayStation 4 only, from Minecraft Marketplace Catalog and some DLC contents from Minecraft: Xbox One Edition that were transferred to it's Bedrock counterpart (for which was affected, click here, then search for "Playable on Xbox Only, Not Purchasable, Not Usable in Multiplayer")

The easiest solution is to add the line "Make sure the Skin, Resource Packs and the World you're playing on does not have Platform Restriction, if either is Platform-restricted, try removing an affected resource pack on the world and / or switching skins / worlds to another that's not Platform-restricted", or just invalidate that and let me know if the license was updated and developers were making Super Mario Mash-up Pack be available to cross-platform multiplayer.