EXCLUSIVE

Solange Knowles ﻿is missing out on the year's biggest music festival -- Coachella -- because her band members were dropping like flies from illness.

Sources close to production tell TMZ ... members of Solange's band started getting sick, and she couldn't find the right mix of replacements. We're told Solange tried out several different musicians, but it just didn't work ... mainly because Solange and co. couldn't figure out how to work in all the new pieces.

We're told Solange didn't feel like she could deliver a great show for Coachella ... and she ultimately decided to pull the plug. Our sources also say she told a bunch of people she didn't want a half-assed show.

Solange -- who released her new album, "When I Get Home" last month -- was scheduled to perform alongside Kid Cudi, Weezer, Tame Impala, J Balvin and Aphex Twin. We're told she's devastated she had to cancel.