You will need to create a new wallet, with a new seed... and select the "Legacy" option.
You don't necessarily need a new seed. You can use an existing seed, click the "options" button, select "BIP39 Seed," and select "legacy (p2pkh)" from the list (or enter the derivation path: for legacy.) You'll have a legacy wallet backed up by the same seed as your Bech32 wallet.
i advise against doing this. this will make it more complicated when restoring the wallet from seed in the future. most users will forget these extra steps. there's also no protection from mistyping the seed mnemonic. so overall you cannot rely on seed backups anymore and will have to take a softcopy backup of the wallet file.
just create a standard electrum wallet with a fresh electrum generated seed.
here's a guide in case the op needs it.