Not knowing the bolt size, and taking a guess, it looks like a 2BA. and the locking nut insert has perished, hence falling off.
These are available from any R/C model shop (R/C helicopters-cars etc.), which is where I buy my spares.
Or buy on-line., see link.
I'd suggest It would be best to replace both Nut & Bolt, as they're not very expensive.
https://www.spaldingfasteners.co.uk/PARENT4671
ZB Custom S10 3+4, DD3 Digi delay, ZB vol. pedal. Peavey Nashville 400