Can a person who got 50 percent marks in his or her 10th and 12th be eligible to get a seat in an NLU after cracking the CLAT?

In terms of eligibility yes you are eligible to write the CLAT and if successful at securing a seat with an NLU, you will be invited to pursue legal studies in their UG program. 

Here is the bare minimum marks required to appear for CLAT under various categories:

• Forty five percent (45%) marks or its equivalent grade in Class 12 in  case of candidates belonging to General / OBC / PWD / NRI / PIO / OCI categories
• Forty Percent (40%) marks or equivalent in in Class 12 in case of candidates belonging to SC/ST categories.