If you wish to use the management shell to create a database these are the commands I use, I also find powershell more reliable than the ECP GUI:
Create Database:
New-MailboxDatabase -Name "DB1" -EdbFilePath D:\DB\DB1.edb -LogFolderPath E:\TL\DB1\
Mount Database:
Mount-Database -Identity "DB1"
Add the Mailbox Database Copy:
Add-MailboxDatabaseCopy -Identity DB1 -MailboxServer Exch16MBX2 -ReplayLagTime 00:10:00 -TruncationLagTime 00:15:00 -ActivationPreference 2
Cleanup corrupt files for the Exchange database:
Update-MailboxDatabaseCopy -Identity "DB1\Exch16MBX2" -DeleteExistingFiles
Clean and update the content index database, which is a DAG is seeded from the primary:
Update-MailboxDatabaseCopy -Identity DB1\Exch16MBX2 -CatalogOnly
Create Database:
New-MailboxDatabase -Name "DB1" -EdbFilePath D:\DB\DB1.edb -LogFolderPath E:\TL\DB1\
Mount Database:
Mount-Database -Identity "DB1"
Add the Mailbox Database Copy:
Add-MailboxDatabaseCopy -Identity DB1 -MailboxServer Exch16MBX2 -ReplayLagTime 00:10:00 -TruncationLagTime 00:15:00 -ActivationPreference 2
Cleanup corrupt files for the Exchange database:
Update-MailboxDatabaseCopy -Identity "DB1\Exch16MBX2" -DeleteExistingFiles
Clean and update the content index database, which is a DAG is seeded from the primary:
Update-MailboxDatabaseCopy -Identity DB1\Exch16MBX2 -CatalogOnly