Bacteria suppliments are garbage. Don't add this to your tank as it will just cause further problems. First thing is first, you need to do a water change of 25% to 50% just to be safe. Second, go out and purhcase an API freshwater master test kit. These liquid test kits allow you to do 100's of tests and are much more accurate and affordable than those lousy test strips. One kit costs about 30$ at petsmart. You will need to start testing your water every day, better each morning and each night, to make sure your water parameters are in safe levels while you do a fish in cycle. You will need to do water changes probably every day depending on your test results until your tank cycles. Make sure you have water conditioner handy. I highly recommend you go to walmart and get the tetra aquasafe large bottle. It's about 12 dollars and you get about... 200-300 gallons worth of water treatment? Just be sure it says AQUASAFE and not EASY BALANCE (which is garbage) as the bottles are identical except for that one bit of wording below the tetra label.
Another alternative is to return the betta or rehome him and do a fishless cycle, which would be easier and more humane for your betta. Another handy tip, if you know ANYONE (besides the big box petstores as their tanks are full of sick fish) with an established freshwater tank see if you can get some used filter media from them. Make sure that the filter media you get is kept in a container or baggy with tank water, never dries out or comes into contact with regular tap water, or gets too hot or cold. Make arrangements for the filter media to be put in your tank ASAP. This borrowed filter media will have a beneficial bacteria colony already inside it, and it's the bacteria colony that determines an established tank. This will help kickstart the cycle and cut your waiting time down.
At this time all you can do is a fish in or a fishless cycle and monitor your tank until it is cycled. I will give you a flink to read over explaining the cycle. I'm sure there are plenty of guides you can research yourself, as well as other members more versed than I on this topic. (Byron or Nickau will probably swim on by for a peek soon, they are very knowledgeable members of the forum).
Water cycle.
https://aquariuminfo.org/cycling.html