I am assuming that you are using the Full Node Multi Coin Adapter.
If you want to keep your wallet unlocked forever, then first go to “Wallets” -> “Cron Job” -> “Time to retain withdrawal secrets” and set it to 0. Alternatively, you could set it to a large number of minutes, after which the wallet will lock again.
Once you’ve done the above, then go to your coin adapter settings and enter your passphrase. Note that the passphrase is different than your RPC password. The passphrase is what you used to encrypt your wallet.
With most wallets you should be able to see if the wallet is locked from the CLI with the getinfo or getwalletinfo command. The command to unlock the wallet manually is walletpassphrase. Try it first from the command line to make sure that there are no errors, then once you know that your passphrase unlocks your wallet, try it with the plugin.
If you continue to experience problems, please let me know which coin adapter and wallet you’re using.