=netday

Try checking the Column Properties for both the columns - Are they both set to "Date" for their Column Type and are they restricted to accept dates only? Are all the values dates in all rows in those columns? I think that error implies that NETDAYS is being given some non-date values, which it can't work with.
https://help.smartsheet.com/articles/2476176-formula-error-messages#toc--invalid-data-type

@jivo they both have to have a property of Date for your formula to work.

The issue is your primary column has a date. You need to create a duplicate column for example:

Create a column named "Tissue Received 2" make sure it has date properties.
Copy and paste the data from the original "Tissue Received" column to "Tissue Received 2"

I would then rename the first "Tissue Received" to something like "Primary Column".

Then remove the "2" from the new column, make sure your formula is updated to the new column.

Absolutely.

As a precaution I would make sure there aren't any automations being driven from the original primary column. If so re-map those workflows to the new column.