Maybe related to https://forum.syncthing.net/t/not-able-to-install-on-linux-mint-20-1? Just a guess, as both topics have been created in the last few days and are related to the same Ubuntu family.
Even if it’s a completely different problem, the same request for details on how you’re trying to install Syncthing and to try using the instructions from https://apt.syncthing.net applies.