Problem setting up file sharing between iMac and MacBook Pro

"Share" in this context means 'make available for others to (potentially) use'. it is a One way Sharing, there is no reciprocity.

When you enable File Sharing, there is a bit more to it. You need to specify WHAT will be Shared, and what [local usernames] may use it. so if the iMac holds the files to be shared, you need to specify what iMac files will be shared and by what specific iMac login-ids or Groups, such as all Admin users.

Then when you log in from a remote computer like your MacBook, the credentials supplied must be those of the same local iMac users for whom you allowed access. If you allowed all Admins, you need to supply a username and password for a specific iMac Admin user.

It is not a requirement, but if you wanted to share MacBook Files from the iMac, you would do the ENTIRE process again, staring form enabling File Sharing on the MacBook, and specify what and with which Macbook-local users.