Runtime Error '13' Type Mismatch

Ok, next question from this clueless idiot:

In the workbook attached (test copy), if I insert a new line into the Current Allocations worksheet, I get a popup with Runtime Error '13' Type Mismatch. If I click debug, it shows the below code, with the piece in bold highlighted yellow. I occasionally get the message when performing other actions within the worksheet as well.

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What is causing the issue here, and how do I fix it? Thanks in advance.

Coaching Database (Copy).xlsm

Excuse my complete ignorance, but how would you insert that into the code I posted before?

Thanks! It now works exactly as I want/need it to.