If you're using an Addon Manager, such as Curseforge, WowUp, Ajour etc, you will need to set TradeSkillMaster Addons to be 'Ignored' to prevent any interference or conflict. The TradeSkillMaster Desktop Application will handle updating and management of the TSM Addons.
Follow these steps
1) Within your addon manager, right-click on both TradeSkillMaster and TSM_AppHelper and set them to be ignored (or do the equivalent steps for your specific addon manager).
2) Go into your addon folder and manually delete TradeSkillMaster and TradeSkillMaster_AppHelper
Note: Your addon folder by default is: World of Warcraft\_retail_\Interface\AddOns or World of Warcraft\_classic_\Interface\AddOns
3) Reinstall both TSM and TSM_AppHelper though the TSM Desktop App by double-clicking each row for the version(s) of the game you play
4) Restart the TSM Desktop Application
5) Make sure the addons are still ignored in your addon manager
6) Restart WoW
Additional information:
The TSM AppHelper addon is used to store pricing data distributed by the Desktop Application. The AppHelper addon does not have pricing data when submitted to Curseforge, so when it is installed and then updated with pricing data by the Desktop Application, your Addon Manager sees the file as 'modified' from the original that they host and will try to reinstall the original with no pricing data. Ensuring the TSM Addons are ignored in your Addon Manager will avoid any reinstall loops or conflicts where you aren't able to see pricing data in-game.