Hi Noelia,
First off, a design with two within-subjects is a tricky beast because you also have to accommodate the interactions between each condition and the subject factor. In order to test the highest-order interaction, the correct error term is W1 x W2 x Sub (where W1 and W2 are your within-subject factors). In order to get SPM to use this, you need Sub, Sub x W1 and Sub x W2 as additional effects in the model. In addition, your Sub factor shouldn't be encoded in a single column, there should be as many columns as subjects in the design matrix.
For more on this, please see my paper (https://www.frontiersin.org/articles/10.3389/fnins.2019.00352/full), where the specification of these types of model in SPM using the Flexible Factorial module is outlined. After this, you may wish to consider whether you want to use this type of design, or whether using the traditional approach of running simple group-level models with contrasts from each subject would be preferable.
Best wishes,
Martyn
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