Problems with Play Store and OK Google

I am having the below issues, despite factory resetting this device several times and even using different google accounts.

Phone is a BluBoo X9 running Android 5.1

Play store will continually stop working and keep saying "authentication required".
When this happens I usually have to clear the cache and force stop, if this doesn't work then I have to UNINSTALL UPDATES. Then it will work for a short time.
The problem is that if I leave it running the old version of the play store without updates, this then causes problems with other apps and services that want me to update the play store.

I am also not able to enable OK GOOGLE voice control. If I go into google app -> settings -> voice -> voice match
The option is greyed out and I cannot enable it.