Sharepoint - How do I create Upload only permission in SharePoint Online?

Try to create a custom permission level with at least add and view items as the following;

To create a permission level

• Open the site collection site (the root site not a subsite)
• Go to site setting.
• below Users and Permission > Click on Site Permission.
• From the above ribbon, > click on permission level to be able to create a new permission level.

For more details check

• How to create a new SharePoint permissions level
• Sharepoint upload permission
• Configure custom permissions in SharePoint 2013
• Understanding permission levels in SharePoint

Create a new permission level named Uploaders and check the permissions Open, Add Items, View Items and View Pages. Then grant the permission level to the users and groups who should be able to upload documents but not edit and delete them.

You can manage permission levels from Site Settings > Site Permissions > Permission Levels