If you want to import a wallet, you should import a BIP32 extended private key. It is often derived from mnemonics.
What is the difference if I import the BIP32 Root Key or if I import the BIP32 Extended Private Key ?
The terminology is a bit confusing. The BIP32 derivation is done using a number, called the chaincode, over a private key (called the master ...
If you already have the seed phrase (aka mnemonic) then all you have to do is import that into Electrum.
Click File > New/Restore (ctrl+N)
Set a name
Select Standard wallet
Select I already have a seed
Enter seed phrase then click option and select BIP39 seed and the other checkbox only if you had used a passphrase to extend this seed
Choose the address ...
Disclaimer: there may exist software that does all of this automatically for you, but I'm going to provide a quick walk-through of the steps, which will allow you to compute this manually.
If you only need to recover the last word, then you're in luck, as there are only a small number of possibilities for what that last word could be. The Bip-39 word list ...