It said to report it to the dev:

composite3.fsh: composite3.fsh: 0(166) : error C1503: undefined variable "sunVisibility"

Let me provide more detailed information.

Environment

• Minecraft: 1.12.7_Fabric_0.16.14
• Launcher: PCL2_CE_Beta_x64
• Iris: iris-fabric-1.9.1+mc1.21.7
• CPU: 12th Gen Intel(R) Core(TM) i5-12450H 2.00 GHz
• GPU: Intel(R) UHD Graphics
• Windows: Windows 11 Home 24H2

How to Reproduce

1. Install the shader pack for the first time.
2. Switch from the default profile to the Low preset — everything works fine.
3. While staying on the Low preset, change any setting or switch to any other profile and then back to Low.
4. The in-game chat immediately prints the following errors:

composite3.fsh: composite3.fsh:
ERROR: 0:103: 'timeBrightness' : undeclared identifier
ERROR: 0:104: 'timeAngle' : undeclared identifier
ERROR: 0:113: 'sunVisibility' : undeclared identifier
ERROR: 0:114: 'wetness' : undeclared identifier

Failed Attempts

• It looked like a caching issue, so I deleted shaderpacks/Solas Shader V2.9.zip.txt and restarted the game — the problem persists.
• I then tried to manually define the missing variables in composite3.fsh, but the shader refused to load at all and threw:
  GLSL source pre-processing failed.

Code I Added

The follow code was producted by AI.

// shader.properties
const LOW 1

// composite3.fsh
#if LOW == 0
    /* Medium/High presets: these uniforms are required */
    uniform float timeBrightness;
    uniform float timeAngle;
    uniform float sunVisibility;
    uniform float wetness;
#endif

#if LOW == 0
    float skyLight = timeBrightness * sunVisibility;
#else
    float skyLight = 1.0;
#endif

yup im having this problem too tell me if you found a fix for now ill use diff shaders