Navigate to Documents > Files > File Cabinet
Navigate to folder SuiteScripts : extensions : SC com.netsuite.poshtheme : PoshThemeExtension@3.2.1 : Modules : WebFont : Configuration
Note: You can isolate the problem into this folder as per the error details in the Extension Manager pointed to keywords [SuiteScript, Configuration, Missing comma on JSON].
Click Edit on WebFont.Configuration.json
Media Item field (file): click Edit
Check the JSON contents and confirmed that the JSON has syntax errors (missing commas) using a JSON Validator/Checker tool
Click Save on pop-up JSON code
Click Save on File Record