I got help from another source. This worked for me:

This is a known issue of Studio, it happens when a formatting tag (e.g., bold, italics) that already exists somewhere in the source column is introduced into the target column via keyboard shortcut (e.g. ctrl + B) or via the Formatting buttons in the Editor view, as shown here: For example, you might notice that you have a source text that has several italics tags in the source column, but when you decide to add extra italics into the target column in a different segment and try to save, this error pops up and you can no longer save. To prevent this from happening, the formatting tag would have to be copied from a source segment containing that tag and pasted into the desired target segment. Thus, our recommendation is that each time you insert a formatting tag with a keyboard shortcut or via the Formatting buttons, please save your progress (CTRL + S). If the error then pops up, you know exactly which tag is causing the issue and you can resolve it by deleting the tag and copying and pasting the same tag from a source segment. In this case, accept the error and go back to the tag affected and delete it (you will know it’s the correct one as you will be able to save) and do the work-around described above from then on. 

I got help from another source. This worked for me:

This is a known issue of Studio, it happens when a formatting tag (e.g., bold, italics) that already exists somewhere in the source column is introduced into the target column via keyboard shortcut (e.g. ctrl + B) or via the Formatting buttons in the Editor view, as shown here: For example, you might notice that you have a source text that has several italics tags in the source column, but when you decide to add extra italics into the target column in a different segment and try to save, this error pops up and you can no longer save. To prevent this from happening, the formatting tag would have to be copied from a source segment containing that tag and pasted into the desired target segment. Thus, our recommendation is that each time you insert a formatting tag with a keyboard shortcut or via the Formatting buttons, please save your progress (CTRL + S). If the error then pops up, you know exactly which tag is causing the issue and you can resolve it by deleting the tag and copying and pasting the same tag from a source segment. In this case, accept the error and go back to the tag affected and delete it (you will know it’s the correct one as you will be able to save) and do the work-around described above from then on.