Hi @Mark Gray
Have you tried to reinstall OneDrive? You can reinstall OneDrive and restart the OneDrive sync client.
And remember to turn off the User Account Control: Open UAC settings -> Drag the slider down to Never notify -> Ok.
References:
https://community.spiceworks.com/topic/2257369-onedrive-for-business-missing-right-click-options
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_onedrivefb-mso_win10-mso_o365b/missing-share-button/22ea3512-a927-452a-b8ff-deae55d96f0c
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