When you switch cassettes, does either cassette go in ok? is the problem with one of the cassettes or with the slot?
If the problem is with the slot, check if the record head is fully in place first before trying to fit the cassette in.
Attach a photo if further assistance is required.
I was hoping for a photo of the OGM bay to see where the problem is, however from your description it seems as though the head unit is engaged when it shouldn't be. With the system turned off, try pushing the head unit back into its Stop position.
Let me know if it moves in for you...
The head unit is the part of the machine that contains the heads, and makes contact with the tape when the tape is playing or recording.
You cannot hold the head back whilst also inserting the cassette?
You may need someone to assist with this.
However, once the cassette is in place, all should be well again.
in my first response I requested a photo so that I could see what is going on.
Without seeing first hand what is occurring, I am unable to provide a definitive answer.
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Ok, then since it is not in the correct position it is likely broken and will require a repair.