Membership Pro Can't assign membership


I am using Membership Pro 2 & am unable to assign membership to users on my site. I am creating a user manually in the back end & once created I add the membership level I have (there is only member level on my site) but once I assign this level & add the finish date I hit save changes & I am sent to an error page (attached), there isn't much detail but it's a HTTP ERROR 500

Have you seen this problem before, I have double checked my settings & have minimal plugins on the site so am unsure if it is a conflict.

I am runing Version of Membership Pro 2

Edit: I forgot to attach my screenshot & I cannot attach now in edit mode so here is a direct dropbox link to it -