Quote from: btc123thatthere on Today at 04:31:21 AM

Running Qt version 5.15.5.

How about Bitcoin Core's version?
The latest Bitcoin Core version 26.0 uses QT v5.15.10.

Quote from: btc123thatthere on Today at 04:31:21 AM

Anyone know why it's not able to use the wallet?

I'm unable to reproduce this in v26.0 with "deprecatedrpc=create_bdb" to create a legacy wallet file.
The wallet loads and I'm able to import a watch-only address.

Try upgrading to the latest version and/or try manually loading the wallet via loadwallet "wallet_name" command.
See if it'll actually load without any error, if not, there may be something wrong with the wallet.dat file or the way it's loaded. (more info on that may help)