1. Go to "Settings" panel.
2. Click on "OP PERMS LEVEL" list to choose the level, usually players choose => 4 to have full permissions.
But you can choose the level of your players. If you don't want them to have all permissions....
Level 1 - Ops can bypass spawn protection.
Level 2 - Ops can use /clear, /difficulty, /effect, /gamemode, /gamerule, /give, and /tp, and can edit command blocks.
Level 3 - Ops can use /ban, /deop, /kick, and /op.
Level 4 - Ops can use /stop.
3. Click on "SAVE CHANGES".
4. Go to "Players" panel.
5. Write the player nickname in the case.
6. Click on "ADD" button to put in the Operators list.
7. See the result. (You can delete an operator with the arrow.)
8. Restart server to affect changes.
If you want to change the level OP permission you can read this following guide:
9. Go to "FTP" panel.
10. Click on edit button for "ops.json" file
11. Change the level for Op permission level.
12. Click on green arrow to save changes and restart server.