Lost trying to activate Office Professional 2021 on machine Frankensteined from an old one

We had a terrible old PC that was running Office Pro 21 with valid key.~
I was given an nicer PC which was effectively a shell with two blank drives and a better spec.

I placed the C from the old PC into a newer build and did a fresh Windows install and the Old C became a slave drive but was not wiped. 

I thought I would be able to install Windows Pro again on the new drive as it was a valid code but it seems not. Perhaps it's for one device only. 

The Old C became the E and is still accessible but I can't use and access the Office Suite and activation states that the code has already been used or is invalid.

I don't know how to approach this. The original code was valid, can the code only be used on one device?

I'm not sure how to resolve this. I would really appreciate some advice about how to approach this as the website doesn't really provide the answer. 

Please do ask if you have require any more info and thanks in advance for any help.