Possibly you are trying to do a "Save-As" while the part is still being accessed by the Assembly file IAM. You would need to close the assembly to do a "Save-As".
Looks like you opened the assembly, selected one of the parts within the assembly to edit and are trying to change the name of the part with the Save-As command. You should be using the "Save Copy As" command for this work method.
If you closed all the files and just opened this single IPT file then your method of the "Save-As" would work.
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