Be aware, if you are short on funds in your eBay Balance to make a refund eBay take those funds from your back up source which is usually your bank account you registered for eBay's Managed Payments.
If you still wish to add funds, which device or browser are you using? The app and mobile browser are sadly lacking in some options, so if using those, switch if you can to a laptop / PC and try Chrome, that works well with eBay.
Then, add funds here:
https://ppcapp.ebay.co.uk/myppc/wallet/list
@nicla_6991