How do I add the + sign for New Opportunities back to the Opportunities Page?

Two possible things

1. Does your user role(s) have the Insert or Delete level access to the Opportunities screen?  If you only have Edit or View access you won’t have the “+” button
2. If the button was removed in error, go to Customizations > Edit Generic Inquiry (must be have access to edit generic inquiries).

   

   Then, on the Entry Point tab, make sure that the “Enable New Record Creation” is selected:
    

   

    

There are a few things that could be causing this.
I’m assuming you were previously using this functionality?
Have any customization projects been published since then that may have affected this screen or the access rights for it?
You will need the “Administrator” role to modify access rights.
 

1. Does the user you are logged in as, have a User Role with “Insert” or “Delete” permissions for the opportunities screen? If not, the button will be hidden.
2. Even if the user has a role that would be allowed to add a new record, is it possible that they have another role with tighter permissions that doesn’t allow it? There are situations where a user may have a role with certain permissions, but because they have another role further down the list of roles (alphabetically), the more restrictive role sometimes takes precedence. This can happen even though most permissive rights take precedence according to Acumatica documentation.
3. Check the Opportunities GI screen to make sure and Entry Screen is selected, and that “Enable New Record Creation” is Checked.
   
   If all these items are in order, the user should be able to create new records from the Inquiry Screen.

I do have access. I have tried this. The Generic Inquiry Screen ID: CR3040PL has disappeared from my database. All I have now is a GI (GI990220). 

Have there been any customizations done to this area of the system?
What version of Acumatica are you on?

I’m running a preview version of 2024R2, and neither the GI990220 GI, or the “Opportunities Details” screen that’s set as the entry screen, appear to exist in my instance.

Correct. The CR.30.40.PL is missing. These GI’s are copied in. I am on 2023R2. In my attempts to correct the error, I added these GI’s. 

Okay, so am I understanding correctly that first, the CR3040PL was missing, then you imported GI990220 as a replacement? 
Is Opportunities Details a custom form screen, or is it another GI?
I believe the only way to allow inserting of an Opportunity from the GI990220 GI, will be to set the CR304000 Opportunities entry screen as the Entry Screen.