If you're getting "invalid phrase" then chances are that it isn't a BIP39 compatible mnemonic...
Try using an offline version of:
https://iancoleman.github.io/bip39/If it also says "invalid" then you can be sure it isn't BIP39 seed... In which case, I'd guess that it was an Electrum mnemonic, as Electrum uses a slightly different method to generate/calculate mnemonic checksums to the BIP39 specification (even though it uses the same word list)...
Download Electrum, when setting up a new wallet select "I already have a seed" and try entering your 12 word mnemonic