Cannot send sms after update to Android 6

I cannot send sms and group messages after update to Android 6. I have the following message when I try to send an SMS: impossible to send the message via “name of sim card”. Error 21. Thanks in advance for any help.

Did you already try to enter a phone number manually and send a text to it? For details of how that may help see here.

If you can still receive SMS you could ask your provider to resend the initial config SMS. Perhaps that does some fixing to the settings.

If not, you could ask your provider for the number of their SMS Center (SMSC). You can then check (or update) that number directly in the settings of some SMS Apps.
If not in the SMS App settings, you can enter that number in the test menu you should get when dialing *#*#4636#*#* - Phone info - SMSC.

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