This got me as well, it's related to the new WTMSIG_BLOCK_SIGNATURES in eosio 2.0 and the requirement to pre-activate the new eosio chain. There is a pull request which includes a simplified version of the eosio.bios contract called eosio.boot which once deployed gives you the ability to pre-activate and then activate the necessary features. Details can be ...
If you just want to run a dapp in sandbox environment you can run local EOS node (nodeos). It will contain only your data and will not sync all blocks from testchain.
You can start by downloading binaries of EOSIO and EOSIO.CDT.
and then simply starting local node through nodeos.
By default local node will run on http://127.0.0.1:8888/