iPad enrollment error

iPad enrollment error

Dear Community!

I enroll 4 devices with DEP. 2 devices enrolled perfectly, but 2 devices not. Their status is: Awaiting User Assignment. When I click the Assign User action, I get the following error message:
Invalid value for the parameter {0}

What do you think could be the problem?

Thans!
                New to ADManager Plus?

                  New to ADSelfService Plus?