The same subgroup size must be used for more than half of the subgroups in order to calculate the between and between/within standard deviations.
Suppose you have one subgroup of size 5, one subgroup of size 7, and one subgroup of size 4. So you have a total of three subgroups with three different sizes. Therefore, each subgroup size occurs one-third of the time and no one subgroup size occurs more than half of the time. You would have to add two more subgroups of size 5 (for example) to create a situation where one subgroup size is used for more than half of the subgroups.
The charts are created, but the MR chart lacks control limits and a center line, and the I chart lacks control limits.