If I negative target the main keywords then what will be happen?

You must not target the main keyword as negative keyword, the main keyword is the main source of conversion, and every seller main goal is to rank your product on the main keyword, so that is totally not good to target your main keyword negative, if the main keyword giving you the spend try to lower the bids and try to rank your product on supporting keywords and get conversion from there amazon will give you the chance to rank on the main keyword as well, but don’t stop the advertising on main keyword.

If you will target your main keyword in the negative targeting it’s not better for you because it will decrease your conversation rate as you know the main keyword is the source for driving the traffic and conversation on the product.

according to what I have experienced, you should negatively target that keyword if you ar not getting any results for a long time.
you can also decrease the bids on that keyword.

You can do negative exact targeting of main keyword in broad match campaign, so that you get clicks on search terms which will include your main keyword. It is not recommended that you do negative targeting of your main keyword in exact and Auto campaign, however you should reduce the keyword bid or try pausing it for some time

Most people are doing it because they want to get more relevant keywords other than the main keyword again and again. So if you want to get keywords from the campaign then you can negative target the Main Keyword.

I recommend you to don’t do negative targeting of your main keywords ever, just lower their bids.

It is not recommended to put the main keywords in the negative targeting. If the main keyword is likely to cost you more and you are not getting any orders, try lowering your bids and trying to rank your product on supporting keywords instead, since this will boost your chances of ranking on the main keyword as well.