Never use Convert methods to parse "unverified" data (such as anything generated by a user, or stored in a text field in a database) - they will throw an exception when they meet a problem and that is what you are getting: the content of the column is not a valid single precision number.
We can't fix that - it's a data error and we have no access to your data - so you you need to code your app to be tolerant of such problems or to report such problems so they can be corrected before the data gets anywhere near your DB and starts doing real damage to your data integrity.
Always convert such input with TryParse methods:
Single.TryParse Method (System) | Microsoft Docs[
^] and report problems instead of continuing blindly!