Have you tried typing in the field, and pressing the enter button to see if the text you typed shows up?
I encountered this while playing around with adding bootstrap libraries to a theme, and the bootstrap css interfered with the input field. I had to add a font size to the field to correct it.
You can also try deactivating the two plugins in the admin panel, and then re-activating them, but since it's the same field from two different plugins, I'm not sure this will do anything, but worth a shot.

Thank you very much, Darryl!

The problem related to Bootstrap plugin. After the plugin deactivated, the problem is solved!

If you want to use that plugin, you can try adding the below css to you custom css for the theme in the admin panel. With that being a plugin it will have the same effect on any theme. You would have to add this to each theme's custom css.

div.tagsinput input {
    font-size: 12px;
}

Thank you very much for your kind support. I'll try it. ;)