I can't select and delete a mask

Layers can also be invisible if they're shyd.  If the little guy that looks like it's peeking over a ledge is blue in the layer panel that means there are shyd (invisible) layers.   The rub is that they maybe shy'd because the author thinks you should not mess with them,  Mess with shy'd layers at your won risk. 

The fastest way if you are new to after effects, you can now contact the author of the template

in the 'Contact Me' on MA website. you can find it by going back to the template preview page,

click on the author name and contact .... I'm sure the author will serve you in minutes because

he knows how the template is constructed. There should also be an instructions manual with

your downloaded package, if it is not enough, contact the author, MA have great support

I don't see a mask, I see multiple selected layers. The easiest way to find masks and delete them is to select all the layers in the timeline and press the U key twice to reveal all modified properties. Masks will show up there. You can simply turn them off or delete them in the timeline. There's nothing to it.

I think you need to spend some time learning the basics. I'd start here: Basic AE. After Effects is a very complex app with hundreds of thousands of workflow options. You can't efficiently learn how to do much in AE without a thorough knowledge of the tools and the UI.