I have exactly the same error, error message says “TXT records can only contain printable ASCII”, and I’m sure my dkim record satisfies this requirement. Would be nice to post solution in public and not have everyone post own support ticket, seems to be common issue
Well, there is no solution. Cloudflare will need to fix the issue and that you can achieve for your account only with a support ticket.
However, that is assuming it is an actual issue and not an error on your side (which your error message might suggest). What value are you trying to add? I could just create a TXT record without any issues. Make sure you do not have any invisible whitespace characters in the case you are pasting the value.