NPT is not included in the Hard coded (grumble grumble) threads and needs to be added to the custom thread file.
Close Alibre before continuing.
Look here C:\ProgramData\Alibre Design\System Files and if you don't see a file called alibre_unicode_custom.thd then copy the file below to this location, then change the extension from txt to thd. It includes all the custom threads I have gathered over the years.
If you do have this file, open it with Notepad and add the following lines to the bottom.
;; NPT Section
;;
*1/16 NPT,27,0.2810,B,0.3182,0.2441,0.2812,A,0.3366,0.2626,0.2996,9
*1/8 NPT,27,0.3390,B,0.4106,0.3366,0.3736,A,0.4292,0.3551,0.3921,9
*1/4 NPT,18,0.4375,B,0.5472,0.4361,0.4916,A,0.5763,0.4652,0.5208,9
*3/8 NPT,18,0.5781,B,0.6826,0.5715,0.6270,A,0.7114,0.6003,0.6558,9
*1/2 NPT,14,0.7188,B,0.8499,0.7070,0.7784,A,0.8868,0.7440,0.8154,9
*3/4 NPT,14,0.9219,B,1.0603,0.9174,0.9889,A,1.0970,0.9541,1.0255,9
*1-NPT,11.5,1.1563,B,1.3256,1.1517,1.2386,A,1.3724,1.1985,1.2854,9
*1-1/4 NPT,11.5,1.5000,B,1.6703,1.4964,1.5834,A,1.7176,1.5437,1.6307,9
*1-1/2 NPT,11.5,1.7344,B,1.9093,1.7354,1.8223,A,1.9578,1.7839,1.8709,9
*2 NPT,11.5,2.2188,B,2.3832,2.2093,2.2963,A,2.4706,2.2966,2.3836,9
*2-1/2 NPT,8,2.6250,B,2.8872,2.6372,2.7622,A,2.9637,2.7137,2.8387,9
*3 NPT,8,3.2500,B,3.5135,3.2635,3.3885,A,3.5884,3.3384,3.4634,9
*3-1/2 NPT,8,3.7500,B,4.0138,3.7638,3.8888,A,4.0889,3.8389,3.9639,9
*4 NPT,8,4.2500,B,4.5121,4.2621,4.3871,A,4.5936,4.3436,4.4686,9
;;
then do a Save As and be sure that encoding is set to unicode next to the save button.
Restart Alibre.
Also have a look
here for my thread file editor which helps with editing/adding threads.