You can run GLM with missing values. However, your GLM model is probably incorrect. If you have one observation of each level of group in each block, then block*group is the residual error term. Thus, your model is overparameterized. Take out the block*group term. If you have multiple independent replicates of each level of group in each block, then you can separate the interaction and the residual. But most people consider block, and especially block*group as random-effect terms. You have them as fixed-effect terms. You will need to read more about use of GLM for random effects before addressing the missing-value issue. If you do decide to explore block or the interaction as random effects, you should definitely consider use of PROC MIXED, especially with missing data, instead of use of GLM.