Thesaurus can not be selected in the dictionary app

I resolved this it this way:

• Temporarily disable System Integrity Protection to be able to remove the dictionary files in the following 2 folders.

Instructions here: https://developer.apple.com/library/archive/documentation/Security/Conceptual/System_Integrity_Protection_Guide/ConfiguringSystemIntegrityProtection/ConfiguringSystemIntegrityProtection.html

• Remove all dictionaries from the Dictionary app within preferences

• Remove all files within the following 2 folders:

/System/Library/AssetsV2/PreinstalledAssetsV2/InstallWithOs/com_apple_MobileAsset_DictionaryServices_dictionaryOSX

/System/Library/AssetsV2/com_apple_MobileAsset_DictionaryServices_dictionaryOSX

• Reenable the dictionaries you require within preferences; these will now be redownloaded from Apple.

• Be sure to reenable System Integrity Protection

Viola!!

Due to its release timeframe, your 16 in Mac Book Pro may be running a special build of Catalina 10.15.1, and that may be a factor in your inability to select that specific Oxford Thesaurus. I am running 10.15.1 on older hardware, and have no problem selecting what you cannot. If the following measures have no effect, you may have to wait for the release of Catalina 10.15.2.

Reboot your Mac into Safe Boot mode. You press and hold the shift key from a power on, and release the shift key when the customary boot progress bar appears. Safe Boot is much slower than a normal boot because it is performing several housekeeping functions, and it is not uncommon for the screen to flash, etc. during this process.

Once you have signed into Safe Boot, try selecting the thesaurus again in the Dictionary Preferences. Note if it is selectable, and then perform a normal reboot. Try selecting the thesaurus in the Dictionary again to confirm whether it does or does not select.

works here ok for me

I am experiencing this exact same issue also on a 16" MBP on Catalina 10.15.2

In Dictionary Preferences, I have checked both dictionaries and dragged them to the top, so that they would be the left-most (after All) available dictionaries on my Dictionary toolbar:

and appear as:

This is also the top down order of a control+command+D, or three-finger trackpad tap to invoke Lookup when hovering over a word.

If you want to look-up a Thesaurus entry, click that toolbar item either before, or after you enter your word in the Dictionary search bar.

Have you installed a third-party Thesaurus dictionary beyond those made available by Apple, for use with Apple Dictionary?