It sounds like you bonded all your funds in your combination stash/controller account, and thus can’t cover transaction fees. Can you check if this is the case? To do so, navigate to the Accounts tab > Accounts. Find the account in question and click the little dropdown arrow beside the account balance. This will show your balance delineated into the portions that may be transferable or locked. It will show in which activity locked funds are locked, such as bonded for staking and/or governance, and unbonding from staking.
To change your controller account, please navigate to the Network tab > Staking > Account actions. From there, select the three dots at the far right of the account. Select “Change controller.” You will need a small balance to cover the transaction cost for this action as well.
Unbonding your funds will free them for use, but you will have to wait until the end of the unbonding period to withdraw funds. On Polkadot the unbonding period is 28 days, on Kusama it’s 7.
Can you let me know if these steps solve your issue?
Another resource, if you aren’t aware of it, for practicing with Polkadot/Kusama staking is Polkadot’s unincentivized testnet, Westend. To navigate to the Westend UI, click the upper left corner of the polkadot.js interface where it says Polkadot version x. Scroll down the drop down menu to where it says “test networks,” and select Westend from the list. You can request free Westend tokens at the Westend Faucet Element chat by typing !drip (account number).