Fabre D'Olivet translated with commentary the Golden Verses of Pythagoras in 1813. Nayan Louise Redfield translated that into English in 1916. D'Olivet provided a commentary for the verse in question:
If thou canst at least: for a most rigid law Binds Power to Necessity
D'Olivet's interpretation of this is:
...Pythagoras recognized two motives of human action, the first, issuing from a constrained nature called Necessity; the second emanating from a free nature, called Power, and both dependent upon an implied primordial law.
D'Olivet contrasts this interpretation with Stoicism and Platonism.
This would be one interpretation of what Pythagoras, or whoever wrote the Golden Verses, meant.
The Golden Verses of Pythagoras. Translated by Fabre D'Olivet and Nayan Louise Redfield. G. P. Putnam's Sons. 1917.