Do you mean you cannot log into OWA for your Exchange Online mailboxes?
Did OWA work for your mailbox before?
Do other users in your organization have the same issue?
If the license is assigned recently, it needs some time to sync and please wait a while then check again. Please also check and make sure the license is assigned correctly.
If the OWA worked for the mailbox before, you can try to log with Outlook for Windows to see whether it works.
Try to disable and re-enable OWA for the mailbox, wait for a while then test again. For more details: Enable or disable Outlook on the web for a mailbox.
Clear the cache, reopen the browser and log again. If available, please also try with different browsers.
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