This is the solution that MH2661 posted (and it works!):-

"If you updated and the NEC eligible accounts are greyed out and say "Box 15: State tax withheld", here is how I resolved that issue. 
Go thru the Print 1099's process again and select to Print 1009-MISC instead of 1099-NEC. 

On the screen where you map accounts to boxes, set all accounts that should be 1099-NEC to "omit". These are the account that were greyed out previously. Save. 

Now go back and print the 1099-NEC. It would let me change those accounts now. 

Hope this helps."