SharePoint help - Internal users can't share one document library to external users why?

Hi 🙂

We have a sharepoint site (let's call it SS1)

In SS1, there are multiple document libraries. (https://XXXXX.sharepoint.com/Product_Information)

in one of the document libraries (let's call it DL1) , internal users can't invite external users.

Error: it is not possible to find an exact match

in another document library, (https://XXXXX.sharepoint.com/Suppliers) at the same sharepoint site (let's call it DL2) users can share to external users.

Any suggestions as to what might be wrong/ have to be changed, so users can share DL1 to external users?

Maybe i can look something up in PowerShell or?

(if it is relevant: users have view-only permissions at DL1 )