I tried to create a new tag called material science, but the server states that this is a new tag and cannot be created at remote mobile site. I want to add this tag because I feel it would be helpful for the site.
Tags on Chem.SE are generally not unilaterally created by one person. If you want to make a new tag, the proper method would be to submit a proposal on meta.
Since your post doesn't contain any rationale for why such a post might prove "quite helpful", I am interpreting this post as solely asking for help on how to create a tag.
If you wish to propose such a new tag please make a new meta post and the community will judge it based on its merits. However, I believe that such a proposal might not be very well-received, considering that the tag materials already exists.