I cannot add one of my customers with the same e-mail address extension to the project.

[Emine Nur Yılmaz]

I added 2 of my customers with the same e-mail address extension to the project, but I cannot add 1 customer. 

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(You do not have permission to invite customer to support project). 

Please check the email addresses and try again.

I receive the warnings above, but I can add different people with the same e-mail address extension.

[Brant Schroeder]

@Emine Nur Yılmaz welcome to the Atlassian community

My guess is that you are only able to add customers who have already been created but you are not able to create customers.  I would suggest reaching out to your administrator so they can verify your permissions.