Install or remove windows 7 or later version greyed out??

[AlexTardif]

Hi folks -

Trying to install Win10 on my Late 2013 MB Pro and running into an issue right off the bat: the checkbox "Install or remove windows 7 or later version" in the very first Boot Camp Assistant screen is greyed out and I can't proceed. Googling this question yielded several other folks with similar issue, but I can't find a conclusive solution. What's wrong here and how do I fix it?

I wonder if it's something to do with APFS on High Sierra?

 

[DeltaMac]

How much free space do you have on your MBPro storage?

Do you have File Vault turned on?

 

[AlexTardif]

How much free space do you have on your MBPro storage?

Do you have File Vault turned on?

Plenty of space - it's a 500GB SSD with almost 400 free. File Vault is off.