You are trying to set the user interface client per user via UI Client Assignment function in Administrator settings, however you are not able to do the assignments anymore.
SAP Hybris Cloud for Customer
This is a system behavior. Starting from release 1808 this functionality was deprecated, since company settings is now enabled based on business roles.
Current assignments will continue to work, but now you can only delete assignments.
You can use company settings by role to configure system differently based on business roles.
"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."
UI Client Assignment , KBA , AP-RC-UIF-ACC , UI Runtime Accessibility Issues , Problem