share variable across stages

Try below as workarounds:

1. Predefine Variables at Pipeline Level
•    You can define variables at the pipeline level (in the Variables tab).
•    These are accessible in all stages, but you can't change their value mid-pipeline and expect the new value to persist.
2. Write to a File and Read Later
•    Use a script to write the variable value to a file (e.g., output.txt) in a shared artifact folder.
•    Publish the file as an artifact in Stage 1.
•    In Stage 2, download the artifact and read the value from the file.

# Stage 1 - Write to file
echo "myValue=123" > $(Build.ArtifactStagingDirectory)\output.txt

# Stage 2 - Read from file
$lines = Get-Content "$(Pipeline.Workspace)\output.txt"
foreach ($line in $lines) {
    $parts = $line -split "="
    Write-Host "##vso[task.setvariable variable=$($parts[0])]$($parts[1])"
}

3. Use Environment Variables 
•    You can set environment variables in a job, but they won’t persist across stages.