We will guide you by OS. Please check the following.
Android
Is the Google Play Store supported?
Devices that do not support the Google Play Store are not covered by Teachme Biz support.
Related: List of supported environments
https://help.teachme.jp/hc/en-us/articles/14572215297305
Is the Google account correctly linked to the device in use?
If the Google account and device are not properly linked, it may cause slow downloads or errors.
Please contact your company's IT department or Google Support regarding the sync status of your account and device.
iOS/Android
Are app downloads blocked by security settings?
Your company's security settings may restrict the use of the Google Play Store or the downloading and installation of applications.
Please contact your company's IT department for more information.
Related
Are there things I should check regarding the device used with Teachme Biz?
https://help.teachme.jp/hc/en-us/articles/900005507766