Hello ,
This sounds like it may be a bug, as you should be able to apply materials to objects at any time inside Twinmotion when Apply to Object option.
One possible solution to try is to reset your Twinmotion configuration by following the steps in our Twinmotion is not visible after splash screen article, which can help resolve issues with the UI not functioning properly. If this does not work, then please submit a Bug report using this form. Be sure to send a copy of your file, as well as any screen recordings or screenshots of the issue as they appear for you so our support team can investigate the issue.
Best,
Raghib