Excel to Outlook Run Time Error 287 & Remove Default Signature

[HTSCF Fareha]

I have the attached Excel macro that has a UserForm that is instigated from a command button. This allows a selection of an email recipient from a list and two TextBoxes to add extra detail. There is then the "Prepare Email" button which does exactly that and populates the relevant areas for review prior to sending. On my home setup running Office 2019 this works fine, but when at work which uses Office 2016, I get the following:-

2021-07-07_16-42-35.jpg

This shows up on this line :-

2021-07-07_16-42-19.jpg

This is part one and my main problem.

The second part is that if a user has a default signature already setup in Outlook, then I would like this to be removed. I have been scouring online and have found mention of

Set oBookmark = objDoc.Bookmarks("_MailAutoSig")

but am uncertain if this is the most efficient way or if it would work in my file. Help for these two things would be very much appreciated.

Thanks!
Steve