Tricks and Treachery are the practice of Fools that have not Wit enough to be Honest.

[Les finesses et les trahisons ne viennent que de manque d’habileté.]

François VI, duc de La Rochefoucauld (1613-1680) French epigrammatist, memoirist, noble

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶126 (1665-1678) [tr. Stanhope (1694), ¶127]
    (Source)

Present in the 1st (1665) edition. A 1665 variant reads:

Si on étoit toujours assez habile, on ne ferait jamais de finesses ni de trahisons.
 
[If one were sufficiently able, one would never do tricks or treasons]

(Source (French)). Alternate translations:

Cunning and treachery proceed from want of capacity.
[pub. Donaldson (1783), ¶80; ed. Lepoittevin-Lacroix (1797), ¶122]

Cunning and treachery proceed often from want of capacity.
[ed. Carvill (1835), ¶68]

Treacheries and acts of artifice only originate in the want of ability.
[ed. Gowens (1851), ¶129]

Cunning and treachery are the offspring of incapacity.
[tr. Bund/Friswell (1871), ¶126]

Trickery and treachery are a mark of stupidity.
[tr. Heard (1917), ¶126]

Guile and treachery are merely the result of want of talent.
[tr. Stevens (1939), ¶126]

Cunning and treachery come solely from a lack of skill.
[tr. FitzGibbon (1957), ¶126]

Tricks and treachery are merely proof of lack of skill.
[tr. Kronenberger (1959), ¶126]

Intrigues and treasons simply come from lack of adroitness.
[tr. Tancock (1959), ¶126]

Cunning and treachery are given rise to by mere incompetence.
[tr. Whichello (2016)]