This is how a product upload entry looks for me, with pricing uploaded separately. The <itemdimensions> tag has all of the unitOfMeasure components; what your particular error mentions. To 2nd what Mr. MacCutchan says, your xml tags are missing the enclosing brackets as well. I was going to simply post the ItemDimensions tag, but I hope seeing a single Message structure is of more use to you.
Edit: I should also mention that your c# code making the request does not appear to have any issues. In my case, I do
feedSubmissionId = SubmitFeed.InvokeSubmitFeed(service, request);
because I store those ids, but I don't recall that being required.
<Message>
<MessageID>10</MessageID>
<OperationType>Update</OperationType>
<Product>
<SKU>007-D15206</SKU>
<StandardProductID>
<Type>UPC</Type>
<Value>086876064216</Value>
</StandardProductID>
<ProductTaxCode>A_GEN_TAX</ProductTaxCode>
<LaunchDate>2020-01-02T10:20:07.2366969-05:00</LaunchDate>
<ItemPackageQuantity>1</ItemPackageQuantity>
<NumberOfItems>1</NumberOfItems>
<DescriptionData>
<Title>Medium Super Stitch Wet Mop Head - 4 Ply Cotton, 5" Headband, White</Title>
<Brand>Rubbermaid</Brand>
<Description>Super Stitch® Wet Mop Head, medium, 4-ply cotton, 5" headband, looped end, for absorbing spills, white</Description>
<ItemDimensions>
<Length unitOfMeasure="IN">5.60</Length>
<Width unitOfMeasure="IN">6.50</Width>
<Height unitOfMeasure="IN">1.70</Height>
<Weight unitOfMeasure="LB">1.10</Weight>
</ItemDimensions>
<Manufacturer>Rubbermaid</Manufacturer>
<MfrPartNumber>FGD15206WH00</MfrPartNumber>
<SearchTerms>rubbermaid, rubbermaid mop heads, mop heads, restaurant mop heads, commercial mop heads</SearchTerms>
<ItemType>rubbermaid-mop-heads, mop-heads</ItemType>
<MerchantShippingGroupName>Default Amazon Template</MerchantShippingGroupName>
</DescriptionData>
<ProductData>
<FoodServiceAndJanSan>
<ProductType>
<FoodServiceAndJanSan>
<NumberOfItems>1</NumberOfItems>
</FoodServiceAndJanSan>
</ProductType>
</FoodServiceAndJanSan>
</ProductData>
</Product>
</Message>