The tax year 2019 minimum OS requirements for the Windows or Mac desktop software editions have not yet been released.
TurboTax should make the desktop editions for 2019 available in mid-November 2019.
See this TurboTax support FAQ for current Mac OS minimum requirements - https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-...
Actually for Mac it has been released. If you run any desktop Mac version on Mac OS X 10.12 then there is a popup message that tells you then you must upgrade to Mac OS X 10.13 or later to run the 2019 software. 10.14 is later and is fine. TT 2019 will be compatible with 10.13, 10.14 and 10.15 (scheduled to be released by Apple soon.).
[Mac OS X 10.12 and earlier are not supported because the Apple supplied development software package and Apple does not support those systems any longer.)
Re: Will Premier 2020 be able to run on MacMojave 10.14.6?
The current TurboTax software is for tax year 2019. See this TurboTax support FAQ for the Mac minimum system requirement for TurboTax - https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-...