It's your wallet.dat.
You can't use importprivkey on a descriptor wallet which is the default wallet type in v26.0.

After creating new wallet in v26.0, even after you downgraded to an older version (that still supports descriptors), that wallet isn't replaced by a legacy wallet.dat automatically.
It's just newly created wallet in those old version that do not default to descriptor wallet.

You can either use the importdescriptors command that I've mentioned which is recommended to use future Bitcoin Core versions.
Or create a new wallet that isn't descriptor (untick the option/set a false flag) in the older version to create a legacy wallet where importprivkey command still works.