If the agents are unable to login into Salesforce Omni-Channel, check the following:

1. Check “Storage Usage” under setup. If the Storage is full, then users won’t able to login into Salesforce Omni-Channel.

Storage issue is a common problem in the Sandbox environments.

2. Check this Salesforce Article for additional possible causes – https://help.salesforce.com/articleView?id=000349123&type=1&mode=1