There's a limit of ~3000 domains, so make sure you're not hitting that. You can also try adding the domain via PowerShell and the Set-SPOTenant cmdlet. If that doesn't work either, best open a support case.
sharepoint allowed domains adding error

i have added domains to sharepoint admin center sharing allowed domains but after some domains adding its getting error "added domain name is not valid " and cannot save
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Vasil Michev 66,171 Reputation points MVP
2022-09-07T06:26:21.977+00:00 Emily Du-MSFT 26,551 Reputation points Microsoft Vendor2022-09-08T08:50:54.67+00:00 I do a test and it seems that I can't reproduce the issue in my environment.
You could sign out of your account from the browser, clear the browser cache, close the browser, wait some time, sign into the browser with your account and then go to the SharePoint admin center to see if the issue goes away in your environment.
If no, I kindly suggest you try the following PowerShell to see if you can add a domain to the Allow only specific domains list.
$creds = Get-Credential Connect-SPOService -Url "https://tenant-admin.sharepoint.com" -Credential $creds Set-SPOTenant -SharingDomainRestrictionMode "AllowList" -SharingAllowedDomainList "crescent.com crescenttech.com"
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