Hi,
This looks like an SCSS compilation issue.
The error is coming from templates/indigo/scss/theme.scss where $link_color2 is being used but is not defined during SCSS compilation.
As a first step, could you try:
Template Options → Advanced → Compile SCSS → OFF
Then save the template settings and clear Joomla cache.
There has been a similar Helix SCSS "Undefined variable" issue where disabling Compile SCSS resolved the problem.
Also, please make sure you are using the latest Indigo 2.1.1 package and the latest Helix Ultimate version included/recommended for it.
Hope this helps.
-Regards.