This always happens when I try to do a mission,

There was an Error!

You lack a sufficient reputation for this Mission.

Can someone help me on how to fix this !

I believe the level/difficulty of mission you can go on is restricted by your reputation with the town in question? (I've thought I was remembering it being like that for a long time but have had no way to check at all since my reps are all maxed.)

You can increase your reputation by doing lower level missions (you can see how much they give in the Rewards section, eg, Esior's haven easy mission base payout is
"Reward: 200 10 " (the 10 is the rep gain)
(Note: base rewards are from Satisfactory result; they halve from a Poor result and double from an Exceptional result)
I think that you also get rep points from doing town quests? (not sure if doing The Plot in the area increases it)