Do you have "Enable Salesforce mobile web" checkbox checked? in your Summer'20 org? Clickpath: Setup> Quickfind> Mobile Administration >Salesforce> Salesforce settings
Note that "Salesforce Mobile Web" and "Mobile Administration" and the "Mobile User" checkbox no longer are present in Setup, in the Summer 22 release. Instead, the user may need the "API Enabled" permission, and may need the mobile Connected App to be assigned. AFAIK the assignment can be done in a Profile or a Permission Set, or access to an app can be be pre-authorized for Profiles in the Manage Connected App area, but access cannot be assigned to an individual User directly. And if logon fails, don't forget (I did, of course): the user may be using their system password instead of their Salesforce password!
To enable this, search for 'Salesforce mobile web' in the Setup Quick Find search box.
Make sure to check 'Enable Salesforce mobile web' checkbox under 'Mobile Browser App Settings':
The 'Mobile User' option should now be available on User Records.
Review below links :
https://help.salesforce.com/articleView?id=enabling_disabling_mobile.htm&type=5
https://help.salesforce.com/articleView?id=000230595&type=1
Hope above information was helpful.
Please mark as Best Answer so that it can help others in the future.
Thanks,
Vinay Kumar
Do you have "Enable Salesforce mobile web" checkbox checked? in your Summer'20 org?
Clickpath: Setup> Quickfind> Mobile Administration >Salesforce> Salesforce settings
Incase this is already checked and if you still can't see, then it is probably because Salesforce Mobile Web Experience Retirement (https://help.salesforce.com/articleView?id=000349471&language=en_US&type=1&mode=1) targeted for Summer'20 release.
Per the help text of this checkbox, this option permits mobile web usage which is retired as of Summer'20.