If the tool runs, but you get a "Request Size Limit Exceeded" error when trying to create ad groups, you might be exceeding the ad group creation limits.

The number of ad groups the Google Ads API lets our system post at one time for shopping campaigns is limited. This error usually happens when trying to split by Item ID (too many ad groups to be created one by one).

Depending on the campaign structure you want to accomplish, you can use any of the other levels to split your campaign. Take into consideration that, since shopping campaigns let you manage bids and performance at the product group level, creating a separate ad group for each SKU may not always be required.

For example, instead of creating ad groups by SKU, you can group by Product Type. It'll reduce the number of ad groups you want to create, which will make easier to manage the campaign and bidding in Google Ads (and Optmyzr). 

If your merchant feed is linked and even then if you encounter an error, this can be because the merchant feed may have products which are pending for approval. 

Smart Shopping Campaigns currently can't be split using the old Shopping Campaign Builder because Google doesn't allow creating multiple ad groups in smart shopping campaigns. When the tool tries to push the new structure, it changes the bids as well, and Smart Shopping Campaigns don't allow any manual bid changes. The Shopping Campaign Builder 2.0 lets you build multiple smart shopping campaigns from scratch.

The 503 error might occur hen we try pulling in logs for you after the changes are applied. When the number of changes are high, the system takes time to show the log and might show this error.
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In such cases, whenever a large number of changes are made, you can choose the option to get an email notification when the changes are applied and you don’t have to wait for the logs on the same page.