If the taxpayer isn't receiving a code, it's possible there is a delay in the MFA system, or it's possible their mobile number or e-mail address aren't receiving the code for an unknown reason. If several minutes have passed, one option they can try is to select the option to "Send another code". They can then select whether to have the code sent to their e-mail or their mobile phone.
We have heard of taxpayers using T-Mobile not receiving MFA codes to their mobile phones. If your client is not receiving the TaxesToGo verification code, ask if they are using T-Mobile. If so, T-Mobile may be able to assist them in correcting this on their device.
Irrespective of the taxpayer's mobile provider, you want to ensure they have created their account with the correct phone number. They may also wish to contact their mobile provider to find out if they know why MFA codes are not being received.