OP_CHECKSIG NULLFAIL with btcdeb
In order to verify signatures, you need to feed btcdeb the two transactions (the relevant input transaction and the transaction spending the input) for it to be able to calculate the signature hash.

Edit: as of January 18 2020, you can now feed it the signature and pubkey as a pair instead. You can also replace the signature and pubkey in the script with mock values (e.g. "mysig" and "mypubkey").

See https://github.com/kallewoof/btcdeb/blob/master/doc/mock-values.md