Dear Customers,
Greetings. From your replies, we can see that some of you are still running to this issue. To better help you, we'd like to double-confirm the following options are selected:
1,
For Company Admin: Please access to Exchange Admin Center (EAC) >
organization >
add-ins >
My Templates is enabled.
2,
For Company Admin: Please access to Exchange Admin Center (EAC) >
permissions >
user roles > in
Default Role Assignment Policy detailed page, check if
My ReadWriteMailbox Apps option is selected.
3, For End Users: Please Access to
Outlook Web App (OWA) >
Settings > Options >
General > Manage add-ins >
My Templates is Turned on.

Please make sure that all of the above settings are configured correctly in your side and feel free to let us know how it goes.
Note: Please also make sure you are using the latest Outlook desktop version. For using the latest OWA, please update your browers to the latest version.
Meanwhile, if the issue persists in your tenant, please provide us the tenant information for further help.
For privacy concern, we will send you a private message to collect it. Thanks for your time and understanding.
Regards,
Rudy