There are currently several versions of Decal available, but only 1 is the supported Release version.

Download Decal 2.5.0.1 Release Version

You should use 2.5.0.1 with ACTool.

There are other beta versions of Decal available, 2.5.0.2 and 2.5.0.3. Feel free to try these if you wish, but be aware that since they are beta versions, bugs/crashes/instability are to be expected and no-one can be expected to answer ACTool problems related to running on a beta of Decal.