Can't click or tab into a field
If you cannot enter data in a field, consider the following possibilities:
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You could be in Layout or Preview mode. To enter data, switch to Browse mode.
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The field could be a merge field. To enter data, choose a different layout from the Layout pop-up menu.
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The field could be formatted to prohibit entry. If your access privileges allow, in Layout mode, select the field. In the Behavior area of the Inspector, for Field entry, make sure the Browse Mode and Find Mode options are selected.
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Allow creation of related records is deselected in the relationship definition.
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The file containing the related data has been moved or renamed, or you don't have access to it. In Layout mode, the field displays <File Missing>.
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You can't tab into fields that are not in the tab order; click instead, or add the field to the tab order.
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The field could be a global field or summary field (no data can be entered in Find mode).
Related topics 
Alerts when entering data
Setting the tab order for data entry