Here is an odd one we have come across on a couple of sites now.
When creating a new account in the Account Maintenance window for Dynamics GP2018 you are unable to select an account Category from the category lookup.
If you select the category lookup a blank list is returned and the only options on the tool ribbon are select or cancel.
This prevents you from selecting a category (which is a required field, making this ever so awkward).
There is a work around for this, the problem appears to be with the smartlists lookup. You can get round it by going into Administration-->setup-->System-->user security-->Alternative forms and reports and selecting windows and expanding financial to Categories. Then setting the lookup to use the standard GP window rather than the smartlists.
This will still return a blank list of categories, however, this standard window has a Redisplay option , if you use the Redisplay option from the tool ribbon it will then populate the categories allowing you to select one.