The STL Check modifier checks an object to see if it's correct for exporting to an STL (stereolithography) file format. Stereolithography files are used in 3d modeling to produce physical models, such as with a 3d printer In this case it is used to check that the model is 'manifold', which is a requirement for your COL3/AImesh.
- use the dropdown menu and scrolldown to select 'STL Check'
You first need to check the 'check' box as in the example. In this case it shows 'No Errors' as the 'Status' box confirms. .You can also check the different types of errors that your mesh could have. Recommend to check, 'Everything' as shown below:
Let's delete a face to show what happens when there is an error. So, rerunning STL Check now shows 3 errors.
In this case it is pretty obvious where th error(s) are. Most of the time they are buried deep inside the mesh. You need to be able to see inside the mesh. If you switch to 'wireframe' view, this will highlight the problem area(s) in red.
Note that every time you move the screen even slightly, the error(s) 'disappear'. This means that the errors will not be highlighted anymore. You just have to keep rechecking the 'Check' box.
Also, never correct the errors with the STL Check 'on'. Note where the error is and delete the STL Check. Leaving STL Check modier enabled while editing can cause issues in the STL Check working correctly.
So, after making the corrections that are needed, and then redo the STL Check process. Repeat the process until there are no more STL errors. This is also a good final check after making any other adjustments to the mesh.