Ah, this only shows up when using shader_set_live because GMLive needs to use GM’s HLSLCompiler.exe (which is in the runtime directory) but LTS2026 added another folder in ProgramData that GMLive-server doesn’t know about.
You’ll want to open the directory where gmlive-server is, open Command Prompt or PowerShell (shift-right-click on an empty spot) and do something like this
.\gmlive-server.exe --runtimePath C:\ProgramData\GameMakerStudio2-LTS2026\Cache\runtimes\runtime-2026.0.0.23
(replace the version number with your own if necessary)