Based on the rest of your comments, it sounds like you may not have originally budgeted for the starting balance on the credit card. If you regularly pay off the card in full, or if you had past debt that wasn't accounted for that you are now paying off, you would need to budget for that starting balance in the credit card category's Budgeted column. This sets aside that amount of money so it can be used towards payments.

A negative balance in the credit card Available column means that more money has left your budget than YNAB expected -- you sent more money to pay off the card than YNAB had available based on budgeting for past debt (the Budgeted column) or through budgeted purchases (the Activity column). The fix, in this situation, is to adjust the Budgeted column so that the Available is zero. This may mean the rest of your budget is now overbudget in the same amount, since you had originally allocated that money that you used for paying off the card to other categories. You'll need to adjust the rest of your budget to fit, which yes, will result in you needing to reduce other categories.

Don't try to add fake money to your budget to compensate, like one of your comments down below said. That won't work -- you only have a limited amount of money, and you want your finances to reflect the reality of your situation. Your starting balance was accurate, and you don't need to reset that to zero then try to fake things in order to get them to match up. You'll only confuse yourself and your recordkeeping, and it won't fix the ultimate source of the problem.