UI ++ Task sequence SCCM

I believe your OSDcomputername wasn't being set because you were using an existing Environment Variable which wasn't being overwritten.
Use a unique variable.

I took your code and changed:

From:
<TextInput Variable="ComputerName"
To:
<TextInput Variable="zzComputerName"

And updated:
From:
<Action Type="TSVar"
Name="OSDComputerName">"%ComputerName%"</Action>
To:
<Action Type="TSVar"
Name="OSDComputerName">"%zzComputerName%"</Action>

Now the log shows the correct computername being sent to TSVar:

<![LOG[ Initiating <TSVar> Action: OSDComputerName]LOG]!><time="12:08:44.000+300" date="12-23-2020" component="UI++" context="" type="1" thread="15676" file="ui++.cpp">
<![LOG[ + Set the task sequence variable 'OSDComputerName' to the value 'LAAR0001234']