[Shot Frame Start: Custom attribute value for "Frame start" must be a valid number??]

I have tried to create a custom attribute but I can't seem to create a new description field or alternative way on inputting data e.g. Time Code. I can only seem to input whole numbers? 

[Shot Frame Start: Custom attribute value for "Frame start" must be a valid number??]

I have imported my CSV file including a column of for Start Frame, I have mapped all of my columns. I want to create a column for timecode so I can sort my shots / tasks but i am met with a notification that stops me from completing the import process. The notification reads: Custom attribute value for "Frame start" must be a valid number? 

How do I create a column for time code? See below desired column layout. 

Time Code | VFX Name | Description | Status 

[Fail to import CSV file... A server error has occurred.]

I have imported my CSV file and mapped my columns. When I continue to the next page, a server error occurs and the process won't complete. Why is this?