Does not looks good.
For the purpose of debug, I suggest we start with the default file path first. Could you remove the you custom file path out of your workflow and make it blank? Then use the expression like this.
CONCATENATE(“https://www.appsheet.com/template/getappfileurl?appName=",“AstrivisCRM-1210465”,"&tableName=",“crm_tasks”,"&fileName=",“Attachments%2F”,"WorkFile_for_Task_",encodeurl([task id]),"_",encodeurl([task title]),".pdf")
Troublesome, but the step by step approach is always the shortest way to reach the goal.
This is expression where the pdf file will be saved to default folder with name of Attachments. Test this out first if this returns right full path. With positive test, then change the folder path part to meet your own and custom relative path. This is proper debug process.