Could you provide some screenshots of your custom permission level?
I have created a custom permission level as following picture shows. With this permission level, users can upload files but cannot delete files in the document library.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.