I have an old Electrum
wallet.dat that is password protected.
I know the password, but the seed is lost.
Can this be recovered?
When trying to open the wallet in Electrum, there is an error:
According to https://electrum.readthedocs.io/en/latest/faq.html
The default wallet file is called default_wallet, which is created when you first run the application and is located in the /wallets folder, inside the datadir.
So, as Claris commented,
wallet.dat is a name used by Bitcoin-core and you can't expect Electrum to read a file created by different software for its own purposes.
See How can I transfer a Bitcoin-qt wallet to Electrum? but take care to understand the implications of importing private keys into a HD wallet which has a recovery phrase. There is a danger that any future recovery process will lose your money.
You could instead download bitcoin core, run it and stop it, then replace its wallet.dat with a copy of your old wallet.dat then rrestart bitcoin core to see if it loads that file.