Barela was convicted of robbery affecting interstate commerce and faces a sentence of up to 20 years and $250,000 in fines.
I assume your issue is that you think this is too high (although I don't see what it has to do with the 14th or 6th Amendments). Fortunately, it's also almost completely unrelated to the actual sentence. The number that was quoted is the statutory maximum for robbery or extortion affecting interstate commerce. It represents the maximum amount that any defendant under any circumstances could receive for one count of that crime. A career criminal who threatened to kill an armored car guard in order to steal $10,000,000 would face the same 20-year statutory maximum as someone with no record who threatened to give COVID to a store clerk in order to steal $90.
In some situations, things like the amount stolen or the defendant's criminal record affect the actual crime the defendant is convicted of. At the federal level, that's mostly not the case. Robbery affecting interstate commerce doesn't have degrees or statutory enhancements. Instead, a judge decides what sentence is appropriate. The judge can, in theory, pick anything between the statutory minimum (here there is none) and the statutory maximum (here it's 20 years). 18 U.S. Code § 3553 lays out the factors for the court to consider.
In practice, federal courts generally sentence within the range given in the U.S. Sentencing Guidelines. While the statute itself doesn't distinguish between stealing $90 by threatening to cough and stealing millions by threatening to shoot, the Guidelines do. Courts don't have to follow the Guidelines range but typically do. If they don't, it's much more likely their sentence will be overturned as unreasonable on appeal. Popehat has a good blog post on the Guidelines, how they work, and why press releases quoting statutory maximums are basically straight-up lies.
Sentencing.us has an unofficial calculator you can use to estimate the Guidelines range for a particular crime. If you plug in 18 U.S. Code § 1951 (which translates to the "Robbery" guideline) and enter in $90 stolen, no weapon used, no threat of death, and no criminal record, then you get a range of 33-41 months and/or a fine of $7,500 to $75,000. This is basically the lowest Guidelines range possible for robbery. For comparison, under California state law robbery is punishable by two, three, or five years in state prison. Robbery is a serious and violent crime, so a sentence of multiple years would not be considered unreasonable. But it takes a lot for the Guidelines range to approach the statutory maximum of 20 years.