You haven't run Bitcoin Core in three years. Your old version (v0.15) is incompatible with the current chain rules. When you install v25.0, your old wallet.dat requires a wallet upgrade to open.
Do not rely on the software’s "backup" feature alone. Manually copy the file: bitcoin core walletdat upd
cp ~/.bitcoin/wallet.dat ~/.bitcoin/wallet.dat.old
Here is the standard method to update your wallet.dat file when upgrading Bitcoin Core to a newer version. You haven't run Bitcoin Core in three years