You are a new user and will have relatively low buying limits. Historically registering a credit card is the most likely thing you can do to get your Buyer Activity Limits significantly increased in the short run. See https://www.ebay.com/help/buying/buying-limits-restrictions/buying-limits-restrictions?id=4012&st=12... for more information and instructions.
You also might be running into your Open Transaction Limits (they used to be described in the above linked page, but got lost in a reformat). To get below your limits you need to pay for the outstanding unpaid items that now count towards your limits, but that might not be immediately possible because eBay counts items that you committed to buy but cannot pay for at this time against your limits:
1. At least some active auctions that you have bid on (including the one you are trying to rebid on). Originally only ones you were in the lead on counted, but it seems likely that since the information was hidden eBay expanded this to count all bids on active auctions and/or any pending Best Offer. To minimize the effect of these wait until near the end of the auction to bid only one time at your TRUE maximum you are willing to pay for the item, and then pay immediately after it ends, to minimize the time it counts towards your limit. You can't pay until you win, so don't use up your open transactions on bids you can't pay for until hours or days later.
2. Transactions cancelled at your request (or due to problems with your address). Note that these also can result in lowering the limit, so make sure you are going to buy (and CAN buy, that the item ships to your location) BEFORE you commit to buying.
3. Unpaid Item Strikes. Again, don't bid/commit to buy unless you KNOW you can and will pay for every item.
To get these and other buying limits raised or removed in the long run you need to establish a pattern of reliably paying for what you commit to buy.
