You can do one of two things:
- Install ROCK on the NUC as it stands - this will obliterate the windows installation. This is the simplest way forward - but you will loose the Windows install and you may not be able to get it back in the future.
- Buy a new small (128GByte or 256GByte) M.2 NVME SSD and swap it with the one already installed in the NUC.
The issues around the choice as to which way to proceed are discussed in the thread at Upgrading to nuc8i7beh used nuc - suggestionsrequested.