Your CYA level is too high. The only practical way to lower it to where it should be is to replace water. Ideally you want to get CYA down to around 50, though you certainly want it under 100 no matter what.
Before you get started on that, lets confirm that you really read 100. That means the liquid level in the sample tube was at (or below) the 100 mark and you couldn't see any sign of the black dot, even if you stared at it for a few seconds.
You might want to do the CYA test again, but this time mix equal parts of pool water and tap water, test that, and then multiply the results by two. That will tell you if your CYA level really is around 100, or is actually much higher. The CYA test will report any level over 100 as 100.