The error message you are seeing is indicating that there is a mismatch between the unit of measure defined on the order line and the unit of measure defined on the product. In this case, the order line is set to "Units" while the product is set to "m²".

To fix this, you will need to make sure that the unit of measure defined on the order line and the product are the same or belong to the same category.

One possible reason for this issue could be that the product you're trying to order has a Bill of Materials (BoM) with a material measured in m². This could be causing the system to automatically set the unit of measure on the product to m².

You can check the settings of the product and the BoM, and make sure that the unit of measure for the product is set to "Units" and the unit of measure for the BoM materials is set to "m²".

If the issue is still not solved, you may check if there is any custom code that modifies the unit of measure in the order line or product, or any module installed that is adding custom logic to the product or purchase order.

Another possibility is that the product is being purchased by another unit of measure than the product's unit of measure defined, In this case, you can define a conversion ratio between the unit of measure of the product and the unit of measure of the purchase order.